This is why the market needs some regulation. Here in The Netherlands, ISPs are required to offer free (as in freedom) modem/router choice. Not only can you replace the router, you can even use your own XGS-PON/AON/etc. SFP(+) module.
For a while I had fiber running through an XGS-PON SFP module in my own Fritz!Box. Now I use the provider's ONT (which is just a fiber <-> ethernet media converter) hooked up to a Unifi Cloud Gateway Max.
Plenty of folks here that have their UDM or OpnSense box hooked up directly to fiber with a Zaram XGS-PON module.
Also, I am sorry you have to deal with caps. Data has been unlimited here ever since we switched from 56k6 to ADSL. (I also have unlimited 5G for 25 Euro per month.)
You could also solve this with competition. If there were 10 ISPs it would be disadvantageous to give your customers reasons to leave you. Why aren’t there more ISPs? Maybe too many regulations. It is trivial to lay cable, except of course all the permits.
Or maybe it's an oligopoly where the incumbents have carved up the market and stopped competing, milking their customers instead.
Broadband is then extra special if you let the ISP also own the infrastructure as everyone has to reconnect their service to every house instead of one company (or forbid, the govt) owning the pipes and several companies competing for providing services over those shared pipes.
Imo the competition model doesn't necessarily (always) work that well for infra.
Because it's illegal to dig up the road without a permit and they won't give a permit to install new fiber when the road is already full of perfectly good unused fiber. They only grant one of those the first time.
Here in Germany ISPs are also required to let you use your own router for free.
So instead of making you pay for that option they increase the base price and the provide discounts if you use the provided router. In the end you still end up paying more with your own router than with the provided one. And will probably have a worse support experience if there are ever any issues.
Do the laws in the Netherlands have teeth against such shenanigans?
Best to ask in some local forums. E.g. the Zaram XGS-PON SFP+ module is popular among Dutch KPN users. There is also some cooperation between e.g. KPN (Dutch ISP) and Zaram to make it well. Also popular is the Fritz!Box Fiber 5090, which comes with a module (though currently max. 2.5Gbit). For other modules, AFAIK they need to be set up with the right slot ID of the provider, etc. But the locals will know.
If you happen to be in the Netherlands, some of the KPN tech staff hang around on the Tweakers.net forums. They help a lot of users there who want to go down this road.
For a while I had fiber running through an XGS-PON SFP module in my own Fritz!Box. Now I use the provider's ONT (which is just a fiber <-> ethernet media converter) hooked up to a Unifi Cloud Gateway Max.
Plenty of folks here that have their UDM or OpnSense box hooked up directly to fiber with a Zaram XGS-PON module.
Also, I am sorry you have to deal with caps. Data has been unlimited here ever since we switched from 56k6 to ADSL. (I also have unlimited 5G for 25 Euro per month.)