I'm in the same boat at the moment, and came here to ask the exact same thing, figured I'd chat here instead of a new thread since it's relevant. I guess the key question is what features do you need and how hands on do you want to be? How fast is your connection? Do you want extra VLANs, policy based routing, VPN etc or just basic connectivity?
Common replacements include Draytek, I also see a lot of mention of Asus and Netgear, particularly if you want to run one dd-wrt or openwrt. I'm looking at WiFi6 models, as I want to futureproof.
I have a Mikrotik running and I'm building out wifi on that, but Mikrotik devices are not for everyone, and historically I've used one for an always-on VPN/roadwarrior. They certainly work, but top out the complexity scale in many regards.
In terms of replacing the router and getting basic nat/firewall connectivity, it's very easy, you just need PPPoE support as mentioned. I had no problem authenticating to the network using the above with password 'bt'
I'm super impressed with the Smart Hub I have to be honest in terms of speed, it's in a cupboard under the stairs and I still get well in excess of 300mbps in my office upstairs. I tried a Ubiquiti WiFI 6 pro and only managed about half that, which I found really surprising. I wan't additional SSIDs and some other features including my preferred secure DNS etc which unfortunately does not appear to be achievable with the BT provided device.
I have FTTP too and indeed the ONT is not something I am aware you can change.