Okay, assuming that you want to be good at this while also being good at other things. If that's the case, CHR-caster seems part of what you need. But you need to be able to do this sans-magic.

From XGtE, the Rogue (Mastermind) at 3rd will all you to unerringly mimic speech patterns and accent as well as giving proficiency with both disguise kit and forgery kit (to help mimic handwriting). It's half of the Actor feat, and gained sooner (except for variant humans). But Actor giving advantage is pretty powerful, so I'm not sure it's worth it.

If you wanted to mimic someone you killed, the Bard (Whispers) from XGtE could also help. At 6th you can steal the shadow of someone who died within 30 feet of you, and as an action transform you to look like them, including information the person would freely share with a casual acquaintance. It gives a +5 on the checks to deceive. But it is magic.

If you're okay with magic and can use UA, the Changeling race from the Eberron 1.1 UA can polymorph into any humanoid your size you've seen. Again, depending on DM disguise might have penalties for different builds (significantly taller/shorter, lighter/heavier, like a dwarf), while polymorph has it covered. With either Actor or Mastermind to get the voice, that's pretty dang good and comes online at 3rd (mastermind) or 4th (actor) level. Also automatically proficient with Deception.

Depending on how your DM reads it, Truesight might automatically pierce all mundane disguises as well as magical - is a disguise a "visual illusion"? I'm probably say "no" at my table, but it's worth checking.

Heading back to the CHR caster, some warlock invocations could be nice, though not as good as above. Mask of Many Faces and Beguiling Influence leap out, but there are others.

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