So I am turning level 5 in my campaign and have been playing an Arcane Trickster with a specific backstory that has him chasing down wizards to steal their knowledge cuz plot. We did flaws for feats and my AT wears glasses that constantly get knocked off his face when he fails. He has ritual caster and a ton of lvl 1 spells for an AT due to racial spells and such. Thematically he is supposed to be 2 parts nerd, 1 part spy. However, as a rogue, I still am feeling like a badass in combat and running into fewer situations where my nerd-ness gets to shine. At 5th level rogue I get uncanny dodge which will make me crazy tanky.  This is not sitting right with me because I feel this character should be a glass cannon in combat.  

I considered taking two levels of Wizard to gain portent because it makes sense in our plot and portent is great.  Unfortunately, the spellcaster multiclass rules are silly and so taking a level of wizard would give me redundant access to spell slots I already have and spells I either already have or can access through ritual caster.  As we have a pretty flexible DM I am considering trying to trade uncanny dodge for a higher level spell slot and one spell for it.  What do you folks think about the balance of this? What level spell slot would you give me in exchange for losing access to Uncanny Dodge forever? 

If you have a flexible DM, why not do the multiclass thing and see if they’ll let you swap out Ritual Caster instead. Then there’s no overlap, and you can still get Uncanny Dodge, just not until after you’ve decided to stop leveling Wizard so you get both you just won’t get UD until you hit 7th or 8th level, so it’ll give you plenty of time to decide.

And because of how multiclassing works, if you take even just two levels of Wizard before switching back to Rogue, you will automatically have higher level spell slots because two levels of Wiz are worth six levels of AT on the spell slot table. 

Thank you for the response, I may be misunderstanding some rules.  It is my understanding that, even if I have a level 2 spell slot from multiclassing, that I still can not know and prepare a level 3 spell if neither of my casting classes could do so - I can only cast a level 1 spell from a level 2 spell slot.

Correct, but you still get those higher slots, so you can upcast those lower level spells that you do know. Basically, if you end up going 18 levels in AT, and 2 levels in Wizard, you would end up gaining a second 4th-level spell slot, 3 additional Cantrips, Arcane Recovery, and your choice of basic tradition feature in exchange for 1 ASI and Stroke of luck. Is that worth it? Up to you. 

That is a good point.... Stroke of Luck is great but also... kinda dumb... 

Portent is amazing and I could have it most of the campaign :)

Happy to help.

Maybe the DM will even let you swap feats so you can get Warcaster.

Highly recommend going Wizard.  Arcane Trickster combines amazingly with wizard levels.  yeah, Ritual Caster would be a bit redundant, but the thing is?  You don't have to put rituals in your spellbook if you don't want.  Alternately, your ritual book becomes your spellbook as your understanding of the arcane improves; a DM flexible enough to let you trade out a core class feature should be flexible enough to let you rework your choice of Ritual Caster into something else.  Heh, perhaps Mage Slayer - a magical nerd thief glass cannon who's stealing arcane secrets would know quite well exactly where to stick a blade to most inconvenience other casters, after all.

You end up with more total spell slots, even if you can't learn/prepare spells for them, you get a big boost to cantrip count, and the wizard spellbook has no janky restrictions on schools of magic.  Your Trickster spells are almost all enchantment and illusion; your wizard spells are whatever you want them to be.  Want Catapult, Find Familiar, Feather Fall, Shield, all that good shit?  Do it up.  Pick up a third wizard level somewhere and you get second-level spells in your spellbook; a fourth wizard level gets you a fourth cantrip and an ASI.  I recommend stopping at either 3 or 4, but yeah.  AT/Wizard multiclasses are *****in'; you won't be disappointed.