Welcome to the party, you’re going to have a blast (and lots of chances to bang your head against the wall)!

Here’s my take on your questions:

1. Doesn’t matter at all where you get a domain. Pick any reliable registrar where the price is right.
2. The top-level domain (TLD) is the domain name’s suffix and you can use any that you like. Some cost more than others, so you may want to compare prices.
3. To start with, anything from within the past 10-15 years should be good enough. There are trade-offs, though. Older machines may use more power, for example, but don’t get hung up on details at this point. Get something cheap like an old laptop with an Ethernet port and you’ll be good to go. Once you’ve done it for a while, you’ll understand any downsides and the next piece of hardware you get will be an even better informed choice.
4. Not at all. Self host as much as you want, but I’d recommend starting slowly. Pick one thing and set it up on your server. Tweak it until it’s just how you want it, makes backups, test. Then add the next thing. Immich, forgejo, and a website are nice easy ones to start with. Communications platforms like chat apps and email are much more involved. Maybe save those for slightly later.
5. For sure! You can use any OS you’d like. I’ll mention that a lot of the very basic tutorials will assume you’re using Ubuntu or Debian, both of which use systemd. For Devuan, for example, you could mostly follow those same tutorials, but you’ll have to fill in some of the blanks. I’ll mention that Linux isn’t the only option, either. If you don’t like systemd, you might enjoy something like FreeBSD. It sounds from your experience like you’ll be able to handle this part.

Whatever you do, just remember that it doesn’t need to be perfect immediately. You’re going to mess a bunch of things up and make decisions you’ll wish you hadn’t. You’ll spend a lot of time destroying and recreating things you’ve already done. That’s the joy of self-taught self hosting!

Maybe I missed it, but I didn’t notice anyone elaborate on what I think is the simplest way to handle this.

You can port your existing number to a VOIP provider (I’ve had good experience with voip.ms and Flowroute). Then you get an ATA (analog telephone adapter), connect to the VOIP service and plug your existing landline phone into it. Secure hand ATAs are pretty reasonable priced. I like the Grandstream HT802.

For learners who are helped by seeing someone think through a problem and explain why they made the choices they did it doesn’t get much better than YSAP. (Here’s the link: ysap.sh)

It sounds like you do want to set the default route to the LXC’s address. Then set individual routes for the local LAN and cluster traffic. The routing table will prefer the more specific route (e.g., if you’re trying to get to 192.168.0.14, it will prefer the route you set for 192.168.0.0/24 instead of the default, which is equivalent to 0.0.0.0/0).

If you can get Debian or Rocky Linux on it, you can likely manage all of the NAS-like stuff with Cockpit.

I like @pgo_lemmy’s answer best, but instead of rebuilding the original system, (assuming you did the default ZFS installation) you can add the bigger device as part of a mirror, let it resilver, install the boot loader, and then detach the smaller device from the mirror. It should automatically grow to the bigger size once the smaller device is removed and the only downtime you’d have is from installing the bigger device. Check the PVE wiki and you should find some details on this method.

Not sure about a PDF, but he sells an epub that’s $15 US.

Michael Lucas released a stellar book on the subject not long ago (https://www.tiltedwindmillpress.com/product/ryoms/). It’s pretty thorough, relatively up-to-date, and (like all his stuff) a fun read. It does a great job of demystifying the mechanics. As far as the implementation goes, I’ll put in another vote for Mailcow, which combines all of the components into a very usable docker stack.

You’ve got some good options, but let me mention that if you dig Odoo, there are alternatives to pretty much all the “Enterprise” apps. Just use the Third Party App Store or browse through the OCA (https://apps.odoo-community.org/shop).

As others have said, ersatztv would be the way to go, but apparently is no longer being developed. I’m still using it, but there was a conversation a few days ago that might have some leads on good replacements: https://lemmy.ml/post/44804563

Yeah, I sort of figured it’d be too much. It looks like there are a bunch of booking plugins for Wordpress. That’s definitely where I’d start my search in your shoes.

I’ve been using Odoo CE for some projects and I know that it has apps for scheduling and booking, but if you want to add this to an existing website, that may be overkill.

What platform does your site use? That may help target some suitable options.

Also, check out this list and you may get some ideas: https://github.com/awesome-selfhosted/awesome-selfhosted?tab=readme-ov-file#booking-and-scheduling