I would not consider this machine done just because the keyboard is broken. It may require a replacement, but that should not cost too much, depending on the model. I would recommend you take your machine to a trustworthy repair shop to get an estimate and diagnosis. I would say it is time to replace the machine when it no longer does the things you need it to do without getting in your way or being an issue. If you can still do the things you want it to do, I think you should stick to it a bit longer.

I use mine with a ThinkPad USB C dock, which works just fine. I also have a workstation with a Dell USB-C dock with ethernet and it works fine with M1 Pro. I would recommend to look for used business docks. I have seen some Elgato or ThinkPad docks for below £100.

Nothing much you can do about it. Over time the keys collect oils from your fingers. For some people it’s not as much of an issue but if you eat while using your computer, rarely wash your hands or have sweaty hands, this will happen quicker. Turn the machine off, get a microfiber cloth and some eucalyptus oil and dapen the cloth with it and wipe the keys. Make sure the cloth isn’t wet so that any moisture gets into the keyboard area.