But that's basically what I've already agreed with

I agreed with you that Mount will probably end up being fifth choice anyway, particularly if we bring in the versatile player I've mentioned.
I think we're arguing over something we largely agree on at this point. My original point was simply that for two midfield positions, most teams have three players taking the bulk of the minutes, a fourth providing proper depth, and then additional versatile/youth options behind them.
If you're saying you don't trust Mount to be that fourth player and would rather have someone more reliable ahead of him, fair enough - I've already said I can understand that. That would leave us with Mainoo, Santos, Tielemans + another midfielder as the main four, with Mount effectively fifth choice and additional versatile cover if needed.
That's hardly miles away from what I've been suggesting
The only thing I disagree with is the idea that we need loads of midfielders because we're competing on four fronts. The examples I originally gave show you don't need five or six specialist CMs all playing significant minutes. You need enough quality and depth to cover two positions without leaving players sitting around barely playing.
So if your argument is now essentially "three main midfielders + a reliable fourth + Mount as additional cover", then we're pretty much agreeing.
Yeah, I get that I said the fourth midfielder is "hardly used", and maybe I could've worded that better. What I meant was they're generally used less than the main three, rather than barely playing at all. And when you actually look at the starts and minutes, barring significant injuries to the main options, I think the numbers back that up.