Drow Rouge/Warlock/Monk aka. Assassin - LF tips, no min-maxing

[Lindean]

So...I'm joining on-going campaign. I've never played 5ed IRL (long live AD&D 2).
I have no clue what rest of the team looks like, I've decided to play as Drow Assassin/Spy (that's probably nothing new here) but instead of going pure rouge i wanted to try Way of Shadow.
I'm starting at level 5. I went with : Rouge 1 -> Warlock 1 -> Warlock 2 -> Monk 1 -> Monk 2 and planning to stick with monk at least for Monk 6 (for Shadow Step).
My stats are like:
STR - 10
DEX - 18
CON - 10
INT - 10
WIS - 13
CHA - 14
Background : Spy
For Warlock i took The Great Old One as Patron (Telepathy seems good), Devil's Sight and Mask of Many Faces.
So here's question : At Monk 4th level, should i increase my DEX to 20, pick a Feat (Mobility for example) or go with 19 DEX and 14 WIS?
What should I avoid to not screw this character up? I'm not min-maxer, i just want this dude to be good at his "job".

[Arkhios]

So...I'm joining on-going campaign. I've never played 5ed IRL (long live AD&D 2).
I have no clue what rest of the team looks like, I've decided to play as Drow Assassin/Spy (that's probably nothing new here) but instead of going pure rouge i wanted to try Way of Shadow.
I'm starting at level 5. I went with : Rouge 1 -> Warlock 1 -> Warlock 2 -> Monk 1 -> Monk 2 and planning to stick with monk at least for Monk 6 (for Shadow Step).
My stats are like:
STR - 10
DEX - 18
CON - 10
INT - 10
WIS - 13
CHA - 14
Background : Spy
For Warlock i took The Great Old One as Patron (Telepathy seems good), Devil's Sight and Mask of Many Faces.
So here's question : At Monk 4th level, should i increase my DEX to 20, pick a Feat (Mobility for example) or go with 19 DEX and 14 WIS?
What should I avoid to not screw this character up? I'm not min-maxer, i just want this dude to be good at his "job".

Just to be sure you're aware that, in 5th edition, Rogue's sneak attack only works with a very limited selection of weapons, and unarmed strike isn't one of them.

[Lindean]

Just to be sure you're aware that, in 5th edition, Rogue's sneak attack only works with a very limited selection of weapons, and unarmed strike isn't one of them.

Yea, i was told by my DM that it have to be Finesse weapon or Ranged attack. I will probably use Shortsword for some time (as it counts as Monk Weapon).

In addition...doesn't Monk Unarmed strike count as Finesse attack due to the same mechanic as finesse weapons?

Edit nr3: Read some posts on forums, and it doesn't count.

[Corran]

What should I avoid to not screw this character up? I'm not min-maxer, i just want this dude to be good at his "job".
Avoid tri-classing (at the very least until you reach the higher levels of play). It will slow significantly you important abilities and your character will feel weak. Just pick 2 of monk, rogue and warlock, and go with that.

[Arkhios]

Yea, i was told by my DM that it have to be Finesse weapon or Ranged attack. I will probably use Shortsword for some time (as it counts as Monk Weapon).

In addition...doesn't Monk Unarmed strike count as Finesse attack due to the same mechanic as finesse weapons?

Edit nr3: Read some posts on forums, and it doesn't count.

Yeah, Finesse is a specific weapon property. Just because a monk is able to attack with certain weapons by using Dexterity instead of Strength doesn't mean they're treated as weapons with Finesse property.

In other words, it's not the same mechanic, effectively. Similar, but not equal.

Effectively it falls into same category as the Shillelagh (druid cantrip) does. Martial Arts effectively grants a Monk special "power" to use a different Ability Score for attack and damage rolls with specific weapons. In a similar way the Shillelagh gives you, for 1 minute at a time, the ability to use Wisdom in place of Strength with a quarterstaff or a club you held in your hand when casting the spell.

[Lindean]

Avoid tri-classing (at the very least until you reach the higher levels of play). It will slow significantly you important abilities and your character will feel weak. Just pick 2 of monk, rogue and warlock, and go with that.

That tri-classing was crucial in my opinion (and i can't change it since character was made, all that left is to pick warlock spells), extra feats from starting as Rouge (and thieve's tool proficiency), Warlock will probably stay at level 2 till the end since i've took it for Devil's Sight (i want to see in my own Darkness spell).

Character was inspired a bit by "wetboy" job from Night Angel trilogy.

[Corran]

That tri-classing was crucial in my opinion (and i can't change it since character was made, all that left is to pick warlock spells), extra feats from starting as Rouge (and thieve's tool proficiency), Warlock will probably stay at level 2 till the end since i've took it for Devil's Sight (i want to see in my own Darkness spell).

Character was inspired a bit by "wetboy" job from Night Angel trilogy.
Ah, ok, I thought you could still change such things. In that case, I would probably do as you said in the op, that is advance with levels in monk so I can get extra attack, stunning strike and more ki points. It's a tough decision whether to prioritize shadow step (monk 6) or cunning action (rogue 2), but I think it's safe to say that I would probably want to be warlock 2/ rogue 2/ monk 6 at character level 10 (given that now you are warlock 2/ rogue 1/ monk 2). If you can pick stuff from SCAG, I would make sure I had the booming blade cantrip, because it plays well with mobile builds or when fighting inside darkness, and it plays well with the advantage from when you spend your BA with shadow step as well. Another invocation I would like would be mask of many faces. Good feature for an assassin (even better if you combine it with the actor feat and even with the cantrip friends; this combination can cause a lot of misdirection and hilarity). Invisibility would also be useful for an assassin type of character, so maybe at some point look at getting a 3rd level in warlock (AFB, but I think warlock get invisibility in their spell list; if not forget what I said).