Wizard Rogue?

[Lupine]

What would be the best build for a Wizard Rogue?
I'm thinking that True Strike plus Sneak attack could be a good combo.

Before you suggest Arcane trickster, I'd prefer an Assassin.

[Frozenstep]

True strike isn't that great. Short range, concentration, and honestly you're better off attacking twice.

Arcane trickster is pretty nice with stuff like booming blade for more damage, but if you're set on assassin I would ask how you plan to play. Do you want to mostly be relying on sneak attacks, or do you want to have higher level spells? Are the spells you cast just in service to being an assassin? Do you want your sneak attack to come from melee attacks or some sort of ranged attack?

[Lupine]

With a few exceptions, I'd like my character to be mostly melee based, with mostly spells mostly for utility. High levels spells are not a low priority, but the optimizing the damage boost from sneak attack is my biggest priority.

[MikeRoxTheBoat]

If you just want the true strike "combo" get magic inititiate or be a high elf/half elf with high elf heritage.

If you really want Wizard, War Magic tends to be good for classes who won't be casting leveled spells often. Only problem is that it clashes with your Uncanny Dodge, but you essentially have a flow chart going for if the Arcane Ward AC would prevent the hit. If it doesn't, then Uncanny Dodge. Shield, Absorb Elements, and ritual utility could be helpful, too.

Another option is be an elf of some sort and go Bladesinger. Usually helps on roguey types.

[Sparky McDibben]

Usually you multi-class because you need a feature or a complement to an existing ability. For example, if you're a rogue, you're probably going to be doing most of your damage with Sneak Attack. Damaging spells will compete with Sneak Attack for damage output, not add on to it.

I would recommend illusionist, probably up to level six. That'll get you some 3rd-level spell slots without crimping your rogue progression too much.

[DeTess]

Taking at least 2 levels in divination wizard could increase your chances of getting one of your assassin ambushes off.

[Sparky McDibben]

Taking at least 2 levels in divination wizard could increase your chances of getting one of your assassin ambushes off.

Good call! There's a ton of things Portent can help rogues with.

[Frozenstep]

If that's what you want, I'd say you only want a few wizard levels, just enough to pick up the utility you want, and the rest into rogue for maximum sneak attack damage and slightly higher hp so you can live in melee combat.

2nd level spells provide you some real useful things like blur and mirror image so you can survive longer in melee combat, or shadow blade so you have advantage in dim light and do more damage. You can also pick up invisibility, which is great for out of combat things and potentially setting up for assassination. You need 3 levels in wizard to get this.

[Man_Over_Game]

Rogues need to attack to maintain value.
Wizards need to cast spells to maintain value.

You can't do both at the same time in almost any scenario (there are a few spells that don't interfere with the Rogue's attacks), so it's best to focus on one over the other. You can go for a lot of Rogue levels and a few Wizard levels, or vice-versa, but I would not recommend both.

If you're planning on being a stealth specialist, I wouldn't recommend Wizard as the best choice. A Land>Grasslands Druid will have a lot more to offer you, including many spells that will make you a better rogue, as opposed to interfering with it (with things like Goodberry, Longstrider, Jump, or all the stealth spells you'll want to care about). Transforming into an animal, or ignoring nearly all difficult terrain, would mean that you'd be able to get into and out of places with ease. While they'll be searching for an invisible assassin, you'll be fleeing as an unnoticed squirrel.

[CheddarChampion]

I'd go Wizard 18/ Rogue 2 for a power build.
All the wizard's spellcasting plus better skills and mobility from rogue. Light armor adds a bit, too.

This would pay off when all the melee enemies swarm you: if you manage to survive you can disengage away and fireball them in the same turn, as opposed to misty step and a cantrip.
Booming blade plus cunning action would be good no-resources-spent backup.
For your spell mastery, absorb elements/disguise self/fog cloud/grease/shield/unseen servant and invisibility.

For the OP, since you want more rogue levels I'd go Assassin 11/ Bladesinger 9:
6d6 sneak attack, assassinate, uncanny dodge, evasion, reliable talent, 5th level spells.
Greater Invisibilty will be your best friend.