You can get the UDM-Pro to run a PPPoE session which replaces teh modem.
That's wrong in several important ways:
* There's no modem with FTTP
* There is an ONT (which arguably does the job of a "modem"), but you can't replace it
You're right in saying that if you bought service from BT, and you connect your own router to the ONT, then it needs to run PPPoE. (Some other providers don't use PPPoE, e.g. Sky and Talktalk resi)
I haven't used the UDM/UDM-Pro, but people on this list have said it doesn't do a good job of PPPoE, so buyer beware.
I would suggest, firstly: if you're not a technical expert, then seriously consider sticking with the BT-supplied router. It's free, it's supported, and it's already set up to provide a gigabit.
If you are a technical expert, then you wouldn't be asking on this list anyway.
If you still want to use your own router, then buy whichever model you're comfortable with configuring. *You'll* be the one responsible for setting it up and tuning your network to make use of a gigabit.
I think that when most people say they want to use their own router, what they really mean is they want better wifi coverage. In that case, in my opinion it makes more sense to install separate wifi access point(s) in other parts of the house, with CAT5e cable(s) back to the router. This will give you far better wifi coverage than any single device in one location.