Just the capacity for network monitoring for troubleshooting makes it worthwhile. Not being able to SSH into Netgear's firmware, let alone having access to tcpdump is an advantage right there.
If you mean a DSL modem or cable DOCSIS, I don't think those are easily replaceable. But you can definitely put an OpenWRT device right behind it and use that. It's pretty straightforward (plug in the upstream side, wait for it to get an address, done).
As for how much you need to know... okay. That's a tricky question because, the most you mess with OpenWRT, the more some stuff becomes automatic, and that makes it easy to forget things. That's not on you, that's on me.
That said, thinking about it a little, the defaults are pretty workable right after installation. You'll have to set an admin password on the OpenWRT box (it nags you until you do these days), which should be familiar. Turning up wifi is a little tricky at first. I would recommend reading through the quickstart guide once or twice before digging into OpenWRT configuration because it lays out all of the basics that you need to get going. It's about as well written and useful as the manuals for access points were way back when.
One thing I would recommend is, if you build an OpenWRT box, setting it up before you plug it in and use it as your network gateway. It's much easier to poke at it without having "When is my network going to come back up?" rattling around in the back of your mind.
If you encrypt your data before uploading it to B2 you should be fine. Unless you run it yourself on your own server, you can't be sure that your data will be unreadable by anyone poking around unless it's encrypted.
I've been using B2 for my offsite backups for a couple of years now (since 2018), and both Duplicity and Restic encrypt data before shipping it over. I wrote about it here.
Glassdoor always seemed like a trap to me. What better way to find (potentially) disloyal people to add to the in-house blacklist or layoff file than to set up a site where you can collect negative reviews of workplaces and sufficient data that identities could be ascertained?
Burner phone. Cheap laptop with nothing on it (like a Chromebook). Burner Gmail address and birbsite profile in case they ask you to log into your accounts. Remember the 100 mile border zone.
If we ever do, it'll be 40 or 50 years from now.