Regardless of whether your ISP is leveraging their ownership of your router to violate your privacy, they are using it to exploit you financially. Owning your own equipment is always going to save you money compared to what an ISP will charge you in rent.

Yes, I only use it via syncthing-fork so this is a distinction without a difference to me.

Not sure if I qualify as extra paranoid but this whole situation feels very sketchy and has me reconsidering my use of syncthing. Making significant changes like this without any explanation is extremely bad practice.