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.




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:
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!