You're demand is really weird let me explain you a bit :
Proxmox and CasaOS are mainly use for servers
MX Linux and Windows are the opposite
So my first question is what will be this machine?
The second important question is why using Proxmox and CasaOS?
Proxmox is a hypervisor (a type 1 hypervisor) basically it's like VMware or virtualbox (that you run as app in your OS), but has the place here as the main OS.
CasaOS on the other side is a bit different, it's more an user friendly self hosting service.
So you cannot really use the both if the machine is meant to be a server.
And lastly sure you can dual boot windows and MX Linux (even if windows will surely eat your MX Linux partition, and wouldn't recommended it).
But again you cannot really use your PC as a server and as a home at the same time ;-)
I dont want that of a beast but only a cheap and decent smartphone that will supported by most of the Mobile Linux Distros to use a phone completely privately instead of using android
But you shouldn't use any apple product, they are as bad for privacy as using Google's one. Do not use an iPhone (lineageos is pretty good to use), do not use Apple's services... And keep going
Don't know if we should impose Signal as an alternative and forget SimpleX, in fact SimpleX is better in many other ways than Signal, the only problems are early dev problems...
Thx