DinoInDisguise
A russian spy disguised as a t-rex.
I do like @DinoInDisguise idea though with magic items. You don't need Epic Boons for the eight psuedo-levels and could instead go "full-immersion" and make all increases in power be diegetic via magic items.
Might turn off multiclassing and turn multiclassing into feats. Maybe the first feat gives you the core features, and the second feat gives you a subclass feature? Will play with it to see.
It is not hard if you pay attention to the bonuses given by the level ups in 5e. They are fairly simple and revolve mostly around ASIs, feats, and proficiency bonus. All three translate nicely to items, and it gives you, the DM, fine control over player power curves.
For example, the difference in PB between 12 and 20 is a +2. So a level 12 with a +3 weapon and a level 20 with a +1 weapon, assuming the same primary stat, should be very close on "to hit" and "damage." Add in armor that gives HP, and a few "feat" items, and you may just end up with something close to where you want to be.
EDIT: Remember that skill checks largely can be ignored, as skill check DC changes are not required.