Hiya.
Yeah, the 20th level "optimization theoreticals" are pretty much pointless. I liken it to a group of 8 year olds standing around the playground at recess, each trying to convince the others of who would win in a fight between Batman and Superman.
Starting at 20th level just to test out your "build" seems, to me anyway, a rather hollow endevour. I mean, the very idea that one "build" could be better at 'adventuring' than another is folly. Any DM worth his salt would be able to present challenging situations of all kinda (each column of gaming in 5e...Combat, Role-Play, and Exploration)... effectively reducing said "build" to the zero-sum game that D&D (and most RPGs) tend to be.
My highest level character was Denakhan. He originally was an Elf and got to 5th level before we switched to AD&D (1e) where he became a Fighter/Magic-User. At about level 8/14 or so, on player was "jealous" of Denakhans ability to wear armor and cast spells, so when it was his time to DM he had Istus (we play in Greyhawk for our 1e to this day) make me choose Fighter or Magic-User. I choose MU. After that, advancing to his highest level of 20th was faster. I think he's about 80k to 100k or so away from 21st. Of course, getting him to level 20 took 6 years of real time, playing an obscene amount of time (kids; no school during summer, after school, holidays, etc....10 to 14 hour sessions were the normal on ever weekend). Back in those days, anyone saying they just "made their character at level 12" (or 15, or 20, etc) would have been laughed at heartily and given "The Black Mark" (meaning that anyone playing with said person was instantly suspect in their RPG'ing pedigree). Hell, starting at any level above 3rd would get you the stink eye...
For the last couple of decades and a half, I (we) haven't really put much stock into character level. We just have fun playing together, eating junk food, laughing and rolling dice. It's generally easier to do that with less "character stuff" (be it system or level) as there is less to keep track of and look over during the game. Highest we've played to recently was a Dark Dungeons (RC/BECMI clone) where everyone got to about 14th level (that system goes to 36th...so I guess 14th is the equivalent of around 8th or 9th maybe). For 5e...highest character so far has hit level 4, halfway to 5th.
^_^
Paul L. Ming