partly because I’m scared people will start blasting RSS spam everywhere and it will be my fault.

That is fair. Might be worthwhile talking to instance admins and core devs about how best to make use of it? Putting it behind some admin approval or administration might be the best way.

There was an instance a while back that was dedicated to something similar. Their system was to define the feeds themselves without any real user input, and it never really took off.

Maybe a dedicated instance that provides more user control but is also set up to control and limit things could go a long way? One basic control might be limiting usage to user accounts older than a certain threshold. lemm dot ee does this for image uploads (4 weeks minimum age).