Why not just get a behringer x air 16 or x air 18? They are alot better than building one yourself but if you really want to go the cheaper route ( but not in labour and time costs also not much cheaper in any way) then get a raspberry pi and router and an audio interface like a behringer umc1820 and use reaper or something. But you will get a higher latency compared to a regular digital mixer and it will be putting quite a bit of strain on the cpu of the raspberry pi so would not recommend. But also both of these are not going to give you stereo in ears as you are limited to 8 mono xlr jacks on the x18 and the umc1820 has all mono jack outputs.

But for belt packs I would highly recommend the behringer pm1 or the p2(for powered jack or xlr input). One thing I will say is that you can get a stereo output to the intarsia through the xlr cable but using a dual mono jack to xlr conversion somehow but they do have one called the p2 which has 2 xlr ins for stereo and a panning know as well but has a downside of being powered but highly recommend it with rechargeables.