This is so funny, I dived into the custom keyboard rabbit hole just 3 weeks ago. Never thought this would be so intriguing and time consuming.

Did you consider Mill-Max Sockets?

My personal favorite are 80% TKL ISO low profile keyboards, which is quite uncommon and I've not found much except the

  Keychron K1 Max 

having this layout. Unfortunately, the integrated Gateron brown/red/... low profile switches are not my thing - my favorite ones would be Lofree Kailh Ghost, but these don't fit the keyboard, although it is hot swappable because low profile switches are non standard (at least 2 different layouts I know of).

So I also thought about a custom solution. I found pretty impressive open source firmware and pcbs [1], but I noticed that I just didn't have the time to do all this. Since keyboards are so an individual device, manufacturing bigger batches is risky and manufacturing smaller batches is expensive... so pretty much no choice than waiting for someone having the same dream as I have or do it myself after shoving free enough time.

1: https://github.com/4pplet/waffling80/issues/1