You can build Infinity from source with your own API key and it works just fine (for now). Someone even made the process close to idiot proof: https://colab.research.google.com/drive/13AE8RvjnCfuBJGaACEqxeBIMo33_l-Sc?usp=sharing

Are you running arch on the metal or through crostini/crouton? The second option probably will only respect chromeos' power management settings, but you may be able to flash a full uefi payload and get rid of chromeos completely with https://mrchromebox.tech/#fwscript

I've had a lifetime sub for NZBGeek (indexer) since 2015 (one-off payment of something like 20-30 USD) and currently pay 6 USD/month for unlimited downloads with NewsGroupDirect. The NGD sub was 4 USD/month until recently and I got it at that price through a promotion that I found out about on Reddit (probably r/usenet) a few years ago.

There's some free trials listed here, but sadly all the best info is probably still on r/usenet.

I've run Zentyal Community on a cheap 1c/2gb VPS for probably 5+ years. Receiving email has never been a problem, and I basically don't get spam. My top tip is to use a unique alias for everything, e.g. thirdpartyname.randomstring@your.domain every single time you have to give your email address for something. That way, when a third party is compromised and your provided alias starts receiving spam, you can just update your alias with them and on your MX. If you deliberately make any of your aliases public, you'll probably want to setup rspamd and postgrey.

Getting opendmarc, opendkim, certbot, etc set up in a way that Zentyal wouldn't nuke it everytime it updated was the biggest hassle for me, and I seem to remember having to open a ticket with Microsoft to get my outbound emails accepted by O365 too. Shouldn't be an issue anyway if you use a smarthost for outbound.

This definitely isn't an endorsement of Zentyal btw. In fact, to be clear, most of my setup efforts were spent fighting its design decisions. I'd recommend that you find something containerised which is well maintained and does only what you need. And put some time aside, especially if you've never set up an MX from scratch before.

It's useful if you've replaced a failing controller or cables, or you're about to finish for the day and want someone else to deal with it when resilvering fails and the drive faults again