If I did live in Sweden and did have Bahnhof, could I hypothetically use my own router by a SFP+ or other connection to the "handoff interface" or whatever people call it these days?
Yes. In general fiber connections here use the "Stadsnät" system (municipality/city network). The municipality owns the cables and provides you with a dumb fiber/RJ45 converter. You hook up your own router (if you want to use one) to that. The only thing your ISP provides is, well, the connection. If you're unhappy with your ISP then you can switch from the stadsnät's website in ~5 minutes. Or at least that's the hap…
They themselves can act as what's called a communications operator/provider which allows you to use various services over dark fiber which is installed in your apartment or house.
In the case where Bahnhof takes over as the communications operator/provider, you are essentially directly connected to their backbone and they can provide all their services, currently up to 10 Gbps.
If you so wish, you can completely avoid any CPE or media converter and hook your equipment straight up to the fiber. Mind you, it's good if you're careful with that fiber since it is terminated in the last 20 cm or so completely without a jacket, so don't break it or damage it in any way unless you want to pay for a technician to come out there and re-terminate it for you.