Can I connect our external FTTP optical fibre to the WAN port on our router?

There's a few things to take into account.

Firstly Most ISP support folks are not actually going to know much more than "we give you a box, we plug it in, and it works" - There's probably a few ISPs that do better than the basics, but mine didn't even know how to configure IPv6 when asked.

I suspect that taking that into account, setting up SFP is going to be interesting and require a few things the average person can't get. The average FTTP connection uses GPON - and you might be able to get a GPON transceiver pretty easily - Just make sure you get one meant for ONTs/ONUs not OLTs.

On the other hand, I can't find any information for the software/authentication side of things. Looking at the guide for one of my local ISPs, and the ubiquitous Huawei ONRs that many of them use suggests there's some form of authentication

Its not clear how you'd set that ID in the dream machine, or how you'd get it from an ISP

I'd say with the right ISP it might be doable, but getting the information you need to set it up with most is going to be tricky