It’s not that it’s closed, it’s more that none of the exiting email protocols support a server which can’t read your email (as it’s all encrypted). They do offer Proton Bridge which you can run locally which will handle all the decryption and local mail clients can talk to that as the would any other mail server.
I don’t know off hand if it supports calendar syncing though.
7 Billion Humans and Human Resources Machine (same developer) are probably the most kid friendly ones I’ve played. TIS-100 I found quite fun, but it’s assembly like programming which might be a hard starting point for kids. Zachtronics also make some good games, but the ones I’ve played are more programming games masquerading as something else E.g. chemistry with SpaceChem.
This is pretty similar to my setup except for using a Valve Steam Link over the nVidia Shield. PS5 controllers connected to Steam Link. Steam Link connects to my desktop (over lan, previously PoE worked well). It works quite well overall. A small number of games seem to have issues with being streamed, but they’re pretty few and far between.
I haven’t played it properly either. But there’s a community mod called Deus Ex Revision (It’s also on Steam). Which improves some of the graphics, and looks to include a bunch of QoL features.
https://www.moddb.com/mods/deus-ex-revision