At this point I'd prefer few keys, just 18 per side for my fingers and a row of 4-6 for each of my thumbs. I'd handle numbers, function keys, volume, and whatever else you mentioned with layers.
Those keys already have their own layers in a lot of software. Having to press yet another additional modifier key destroys usability and muscle memory.
For me it’s the opposite: I get RSI from having to stretch my hands to press 2-3 modifiers simultaneously, whereas moving my hands a bit is no problem.
If there are different preferences for arrangement of keyboard layout, then my vote goes to: it would be amazing if there were a laptop where such things could be customized.
Then the people who want niche 40% or 36-key layouts can go with that, and those who prefer more keys can go with that.
IMO, the row-stagger is an unergonomic, archaic skeuomorphism. This is also a niche opinion.
Unfortunately, I'm sure there are practical reasons why "modular keyboard" can't happen (not enough market interest, strength of the laptop hull suffers if the keyboard is a separate module, laptop couldn't be as thin, etc.).