Used to work there. It's fine, esp on the newer builds which you'll be in. XGS-PON and usually plenty of backhaul capacity. Router's crap, chuck it and buy something decent, same as any other ISP.
Yeah. They won't support it if you have a fault so keep the default in a box somewhere, but it's officially kosher to provide your own. You'll get a box on the wall (ONT) that turns PON fibre into gigabit ethernet, and that you have to use. But it's essentially a media converter. DHCP is all you need, no authentication/login - your ONT authenticates you.
I dont know about Gigaclear, but I'm with Giganet and they've been excellent customer service wise and QoS wise. I can't really speak to their supplied router as I use my own opnsense box.
Partner's parents are with Gigaclear. Think it's all been pretty straightforward. They aren't at all techy, so haven't swapped out the router or anything. As someone else said, the supplied router isn't great - it's quite simple in terms of configuration. Not sure about DNS management, but I know there are other annoying things you can't do (some config around naming two WiFi bands).