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Is this where the source code is supposed to be ? https://github.com/ProtonMail/inbox-desktop
My guess is that a 2015 Macbook Air is probably not going to run a MacOS version that is still supported by Apple. That would be yet another reason to simply install Linux. Before you do so you can go for https://rescuezilla.com/ and do disk cloning to an image that you save to some storage like a USB disk. If you do the same after your installing and tweaked Linux installation, you can have the best of both worlds whenever you need it.
Depends a bit on what you’d like to achieve. I’d say play around with YunoHost on a VPS to get started and learn more. With YunoHost you will get XMPP and email server after installation, and then you can start installing other YunoHost apps.
Looks like a nice project if you manage to run your own gateway. People have been posting about this one on Lemmy before https://lemmy.ml/comment/376398 and maybe the software code author is on Lemmy as well : https://lemmy.ml/post/2045121
Maybe this can help after installing pi-hole : https://www.pivpn.io
UPS, good idea.
backups too.
Nice that you are using FSearch :) I would put more excludes in it when you really want to index / In fact, apart from /home I would not index anything else than /etc /usr/share/doc and maybe /var/run/media or /media (depending on which Linux distribution you are using, for example Arch Linux will use /var/run/media and Ubuntu will use /media for removable devices).