At 5 votes per, even if the largest 2 accounts split their stakes into 2-3 accounts, they wouldn't be able to solely control the entire 20 by themselves

And if they split into 4 account instead of 3, then they could. This is not a solid foundation upon which to build an entire voting system, nor an argument for a voting system.

BTW, are you aware that many if not most of the largest stakeholders already have multiple accounts? They mostly use the proxy feature and one set of votes, but that could easily change even without moving stake around at all, and it can also be moved.

I am claiming that having more people deciding who the top 20 witnesses are is better than having less people deciding who they are.

And I am claiming that all voters have significant influence over this even today. Having a large number of votes helps this not hurts, as I said, because even medium and smaller stakeholders can vote for #21, #22, etc. and try to push them into the list, vote for #20 and try to keep them in (or equivalently not vote for #20 and thereby try to push them out), while also continuing to support their favorite choices.

Expecting the largest stakeholders to not have a huge, huge say over the top 20 list is unrealistic no matter what. These are and always will be arguments over the margin. I'm reasonably (though I'll admit not 100%) sure that approval voting with a large (or unlimited) vote limit does better on this, not worse.