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 happy case. Cable (Comhem) and ADSL (usually Telia) are the same shitshows as everywhere else, and there are a few fiber providers (such as Ownit, which my HOA is stuck with) that opt out of the stadsnät system and instead trick HOAs into signing multi-year exclusivity contracts that somehow manage to make Comhem look good.
Interesting thought. If the "router" in this case uses standardised protocols and authentication (ATM?/PPPoE?) you could likely hook up a computer or PC directly, install OpenBSD (etc) on it and have a proper firewall directly in place. :)
I'm not 100% sure but I think so. There's a fiber coming into a wall-mounted non-wireless router. I think I could replace it with any fiber-to-ethernet converter and then hook up my wireless router to the ethernet port.
Or at least that's the happy case. Cable (Comhem) and ADSL (usually Telia) are the same shitshows as everywhere else, and there are a few fiber providers (such as Ownit, which my HOA is stuck with) that opt out of the stadsnät system and instead trick HOAs into signing multi-year exclusivity contracts that somehow manage to make Comhem look good.