My setup is smaller, but when my venerable old router died about a year ago, I acquired an Asus TUF-AX3000_V2 where I installed FreshTomato. One can login via SSH and dump all settings for backup. Likewise, individual or all settings can be done on the command line instead of the GUI. I have a script on my computer that reads CSV files with MAC addresses and more to apply changes in an automated way.
KDE Connect has been mentioned before. You can supplement this and other tools by using a VPN so that both endpoints can see each other even if the underlying network does not allow this. My preferred solutions are Tailscale (managed, cloud-based) or Headscale (for self-hosting).
Yes, XMPP with proper TLS on the server side and Conversations or one of its forks (preferably fetched from F-Droid) using OMEMO encryption should be good enough. If you are brave or paranoid, give Tox a try: https://tox.chat/
Maybe the first question is what your budget is, both regarding money and time. For example, you could buy a pre-configured NAS from Synology or QNAP, which requires less technical skills but more money, or a home-made solution reusing used components (but fresh disks for reliability). Depending on your electricity costs, you may want to choose a low-power solution or something which you power off when not used. For storage, maybe a three-disk RAID5 is a good compromise. For backups, plain S3 cloud storage encrypted via restic is a good idea.
Well, you have Finland in the north-east, Ireland in the north-west, and every land border faces a Euro-zone country. Few other countries can claim the latter.
If at all, you want to use Gentoo’s ebuild system, which can be seen as some kind of superset of PKGBUILDs. I guess one could write a Python script that “dumbs down” ebuild scripts to PKGBUILDs for simple packages (excluding complex stuff like kernel, KDE, …). The main challenge, as pointed to before, would be maintaining a table mapping package names between distributions in order to get the dependencies right.
The “C” in the progress bar is alternating between “c” and “C” to give the impression of munching.
ArchLinux’s pacman with ILoveCandy option enabled.
If you do not trust Tailscale as a company, here is an open source re-implementation of the server called headscale. Some/all clients are open source as well. So, you can review all components yourself or pay for a professional third-party review. Otherwise, if you take a binary blob from any origin, including Tailscale, and have it run with privileges on your server, there are few limits on what this blob can do. Yes, backdoors are technically possible, but probably bad for Tailscale’s business if that ever came to light.