That ONT is going to be a managed network endpoint. Their head-end system talks to it and sets up the configuration for the optical pathway. It is essential that it is not removed. If you replace it with something else then you will not get service.
I believe that you should be able to replace anything else downstream from the ONT. So that router can be replaced with whatever can handle the traffic flow. If it were my setup then I would replace it with my pfsense firewall box.
By the way, is this the kind of fiber cable you were thinking of? https://a.co/d/iJ0guG1
The cool thing about 1310nm light is that it will pass down either multimode or single mode fiber.
At that short of a distance, you should be able to use MM 1310nm SFPs.
As long as the SFPs can do the signalling speed needed by the link then I see no reason why it shouldn’t work.