I can't speak to the aluminium case Q series Keychrons, but my plastic case K4 Pro is very good for a quiet keyboard when you put in good silent switches in it. It has a silicone pad in the bottom which helps a lot with that. There is no need for opening up the case for most any reason.

It is a 96% 1800 style layout, so you have a numpad (albiet, right handed)

All Keychron keyboards support Macs and Windows. The ones that support customizing what the keys do, like my K4 Pro, use the VIA software, which to my knowledge is the only such software that's cross-platform for Windows, MacOS, and Linux.