blade singer, multi-classing and spells

[Night Eternal]

I'm a duel wielding blade singer and once i get to 14th lvl i'm not really interested in the spell mastery compared to multi classing into another class. I wanted make sure that i was correct in assuming that i would lose access to 8th and 9 lvl spells.

On the other hand what would be the best for multi-classing. I play as a stealth dex melee fighter and focused mainly on defense and infiltration spell.

I was thinking about multiclassing but almost every class would be helpfull.
a fighter for action surge and fighting style
a sorcerer for meta magic and either shadow origins or phoniex.
warlock packblade or the new hex or ravenqueen archetype
even a rogue for extra sneak attack damage and assassination.
ranger for increased damage

my stats

strength 17
dexterity 20
constutution 18
intelligence 20
wisdon 17
charisma 18

im lvl 5 and where are in a power campaign. i rolled extremely well and we can reroll 1 and 2. Please no comments on this. I get alot of people don't like this but it works well most of the time and we have a tpk already in one campiagn and 2 players have already died in this campaign.
spell

i don't remember all my spells i know i have shield, absorb elements, green flame blade, message, minor illusion, haste, fireball, alter self, suggestion, invisibility, disguise self, scorching ray. i cant remember the rest right now, i think there only a couple more. But i could use help choosing spell from here on out. I mostly focus on defense spells would like to add some damage ones. would like to get blink and vampric touch

[Citan]

I'm a duel wielding blade singer and once i get to 14th lvl i'm not really interested in the spell mastery compared to multi classing into another class. I wanted make sure that i was correct in assuming that i would lose access to 8th and 9 lvl spells.

On the other hand what would be the best for multi-classing. I play as a stealth dex melee fighter and focused mainly on defense and infiltration spell.

my stats

i don't remember all my spells i know i have shield, absorb elements, green flame blade, message, minor illusion, haste, fireball, alter self, suggestion, invisibility, disguise self, scorching ray. i cant remember the rest right now, i think there only a couple more. But i could use help choosing spell from here on out. I mostly focus on defense spells would like to add some damage ones. would like to get blink and vampric touch
Bolded part screams "Rogue" imo. You get light armor, more damage, Expertise in Stealth and any other, Cunning Action for extra mobility (coupled with Haste), and you can go Swashbuckler to get a free disengage.

If you have high enough stats, other good choices would be Vengeance Paladin (healing spells, smite spells, divine smite, advantage against an enemy), Draconic Sorcerer (metamagic your Wizard spells, better Fireball or Lightning Bolt, better armor and HP), Battlemaster Fighter (slightly better damage, Action Surge, manoeuvers) or Fiend Tome/Chain Warlock (if you have a 2nd or 3rd spell you want to use as often as possible), even Long Death Monk (if high enough Wis, Unarmored Defense can be good. You can also wield your quarterstaff focus with DEX and get THP on kill).

But first, consider this: Wizard's 18th level gives you free 1st and 2nd level spell: free Shield, free Magic Missile, free Shatter, etc...
20th level gives you exactly the same as would Warlock 5: 2 3rd level spells on short rest: Haste and Fireball?

If you are sure you won't ever reach 20th level and you don't care about high level spells anyways, then by all means multiclass. ;)
Otherwise, maybe stay as you are.

Also, I don't quite see your interrogation about spells. You can grab both Blink and Vampiric Touch at lvl 6 and 7 respectively, both are on Wizard's list?

EDIT: My gosh, didn't pay attention to the stat block, it's godly!!!
Then please apologize this small madness proposition:
- Swashbuckler 3: Expertise in Stealth and Athletics (for shoving when necessary), +4 to Initiative, free disengage against enemies you attack.

- Paladin Vengeance 3: smite, Bless, Shield of Faith etc...
OR
- Long Death Monk 3: Unarmored is lesser than Mage Armour, but you can dual-wield with your staff and a dagger, and you get THP (of course if you have Warcaster or plan to take it, this is not optimal suggestion).

Should be fun.

[Clone]

Are you sure it'll be running to a super high level? If you are unsure, or at least want to be more lethal for the time being, take a single Fighter level for Two-Weapon Fighting Style.
You already have your Dex and Int maxed, with a very high Con, so your stats aren't an issue. Pick up Warcaster at the next ASI and see how you go. This brings in the strats of Booming Blade on AoO and casting spells with your swords out.
Then at level 14, you'll be doing 3d6 + 30 a round, not even taking into consideration the crazy magic short swords you may come across. You can level further for Action Surge and then battle maneuvers, maybe samurai from UA for extra protections in the middle of things.

Otherwise, if this doesn't take your fancy, I'd go Paladin all the way even for two levels. Smiting all day and getting healing spells, along with a fighting style (though sadly not Two-weapon fighting) kinda is crazy.