

When you say your talk talk hub is bottlenecky, what does that mean? Are speeds slower that your service contract? Are those speedtests over wifi or LAN? Also which hub do you have? I ask because this site does have good things to say about the newer talk talk hub.
For your questions though:
The modem with authenticates with the ISP and then has ethernet ports to plug into. Some modems have built in wifi which will let you get away with just one device assuming your dwelling is within the wifi coverage. If a modem doesnt have built in wifi then you’d need a router for wireless access.
A separate modem + router will generally have better features on the router, but typically cost more money and may be overkill depending on what you need
I noticed in that article i linked it said that some talk talk hubs have a pass through mode. So if the issue is just the wifi access then you could get away with just getting a router (keeping your current hub).
I selfhost navidrome for the music streaming (+symfonium app for mobile). Multi user and multi library support.
For music tagging itself ive used beets, picard, and kid3 (kde). Currently I am liking picard the most. It took a little bit of learning but less than beets