Advice on levels and feats for my rogue/warlock

[Stuzz]

A bit of backround: this is my first campaign of DnD, I joined the campaign late and begun my character at level 8. He is a level 9 lightfoot halfling Assassin Rogue, and next level (5k xp) he is going to multiclass as a warlock. I'm going to use the unearthed arcana Raven queen pact. His stats:
Strength 8
Dexterity 18
Constitution 15
Intelligence 14
Wisdom 15
Charisma 15
I didn't really understand the game when I created the character, I would have given him 16 charisma if I did. His current attuned items include an ioun stone of agility, a homebrew rapier and a homebrew hand crossbow. The rapier is +2 and does 1d8 and 1d4 damage, and the d8 heals me. The crossbow is a +2 and does 1d6 and 1d8, and can cast lightning bolt twice per day from it. I have the crossbow expert feat.
I intend on taking the booming blade/green flame blade cantrips. I was going to take the mobile feat so I could run up, booming blade then move away with no AoO, then bonus action crossbow instead of cunning action disengage (I know that this isn't exactly how it would work, but we are kind of house ruling it, our DM gives us greater CR monster than our level shows, but we have slightly improved abilities like doing this). However I feel like this could undermine crossbow expert. The rest of the damaging cantrips seem like a waste.
The next problem I'm having is with the pact boons. I'm stuck between pact of the chain and pact of the blade. I already pretty much have a familiar due to Raven Queen Pact; I can have the raven take the help action giving me sneak attack, and if it dies I get advantage for 24 hours on whoever killed it. But chain offers an invisible helper with improved abilities, on top of the raven.
Blade was my initial plan for thirsting blade, but after realising how powerful booming blade can be I'm leaning for towards eldritch smite. If I use it on the crossbow I can knock flying enemies prone and to the ground, or if I surprise I can use it to get extra crit damage. But it takes away from using booming blade, I feel like I can't have both. Does anyone have a solution for these?
Any other feats or ASI ideas could help too, if I get something better than ioun stone I might want to take +2 to dex. I could increase charisma, maybe with actor feat or +1 to wis/con and cha. I considered resilient with constitution to gain proficiency in saving throws. Sharpshooter is also possible for 120 max crossbow range and +10 to low a.c. enemies. Skilled (as I'm not proficient in perception or investigation) War caster, mobile,observant, mage slayer and skulker are all considered too. If I go 12 rogue/8 warlock I can get 4 ASI, or 11 rogue/9 warlock for an invocation and 5th level slots. I wanted to go 11 rogue for reliable talent and the sneak attack dice, and invocations were going to be devils sight, mask of many faces, ghostly gaze and eldritch strike. Any suggestions/solutions for feats, levels and invocations.

[Coec]

I'm literally doing this very thing only from level 1, now level 4 rogue/1 warlock. I'm an assassin but instead of raven queen unearthed arcana (thought about it) I took hexblade.

My first feat was lucky do I get to reroll 3 ability/attack/saving throw checks a day. And with a lightfoot halfling if I roll a 1, I can reroll that as well.

I've found, as I wanted to be a melee rogue, that I'm more and more staying ranged, so green flame blade is doing nothing for me right now. But hex is awesome when I remember to use it.

Based on the graft you mentioned, crossbow expert, it sounds like you're going ranged with melee as back up. Future feats I am considering is alert for that extra +5 to initiative for assassinate purposes, mobile for movement as a halfling only moves 25', sharpshooter, and second chance (XGtE), because I'm all about the rerolls.

For your invocations I've considered for myself devil's sight and mask of many faces. I too am torn by either blade or chain pacts because you can take thirsting blade invocation for an extra attack and that's one more chance yup get sneak attack. However I'm personally leaning towards the imp because I think it would be fun just to sit there and talk to myself (imp is invisible lol).

Mechanically I should go blade but I personally like the flavor of the imp. And he can assist I scouting and help actions, plus free poison (if i remember correctly, AFB, because its a sting attack which means i can harvest it?)

[Stuzz]

I'm literally doing this very thing only from level 1, now level 4 rogue/1 warlock. I'm an assassin but instead of raven queen unearthed arcana (thought about it) I took hexblade.

My first feat was lucky do I get to reroll 3 ability/attack/saving throw checks a day. And with a lightfoot halfling if I roll a 1, I can reroll that as well.

I've found, as I wanted to be a melee rogue, that I'm more and more staying ranged, so green flame blade is doing nothing for me right now. But hex is awesome when I remember to use it.

Based on the graft you mentioned, crossbow expert, it sounds like you're going ranged with melee as back up. Future feats I am considering is alert for that extra +5 to initiative for assassinate purposes, mobile for movement as a halfling only moves 25', sharpshooter, and second chance (XGtE), because I'm all about the rerolls.

For your invocations I've considered for myself devil's sight and mask of many faces. I too am torn by either blade or chain pacts because you can take thirsting blade invocation for an extra attack and that's one more chance yup get sneak attack. However I'm personally leaning towards the imp because I think it would be fun just to sit there and talk to myself (imp is invisible lol).

Mechanically I should go blade but I personally like the flavor of the imp. And he can assist I scouting and help actions, plus free poison (if i remember correctly, AFB, because its a sting attack which means i can harvest it?)

I didn't even think about harvesting the poison, that's another thing for me to consider on the chain side. Could be potent for those extra dice on a crit if I can infuse crossbow bolts with them, like eldritch strike but I won't have to use a spell slot..

[Vogie]

You don't really need additional charisma, because most of your damage is coming from your items and attacks

The only reason you'd want to go blade is for the additional attack and smite route. If you're intending on using the spell slots for more than smites, you probably don't want to go that route. You already have +2 weapons, so you don't need to pick up improved pact weapon... unless, of course, you want to also be able to summon a +1 heavy crossbow or +1 Longbow at some point during the future. Improved Pact Weapon also gives you 1/3rd of War Caster as a bonus - you don't really need the ability to react with spells, as you'd likely respond with a Rapier Strike and if you're dancing around combat, you wouldn't really need the concentration bonus either.


Otherwise, the invisible familiar using the Aid action to generate advantage each turn is probably better for reliable sneak attacks, doubly so if you can harvest poison for arrows.

Observant is probably the best all around option for feats. I wouldn't pick up Sharpshooter unless you find you're doing most of your damage at range.

The Blade Mastery feat (UA) will give you advantage on AoOs with your Rapier automatically, as well as +1 to melee attack rolls, and ability to gain +1AC as a reaction.

Tomb of Levistus may be a decent option to pick up as it is effectively a Legendary Resistance
Misty Visions would probably be very useful - disguise yourself as a bush, part of a wall, a tree, or even be able to create the image of yourself elsewhere to throw off enemies.

[Coec]

Since I'm building the same concept (slightly different), I'm curious as to why observant is such a great great?

[Vogie]

Since I'm building the same concept (slightly different), I'm curious as to why observant is such a great great?

He was worried about his perception & investigation skills, and observant gives a +5 to both. I actually thought he was an Inquisitive rogue, who have several skills that scale with Wisdom in addition to the norm, but it's still a decent feat for anyone who wants to be good at those skills.

[Coec]

I see. I viewed that as he didn't have perception or investigation as a skill he is proficient in therefore he doesn't get his proficiency bonus. Adding observant helps but not in the long run imho.