you could go to a shop and have just a spot fix blend
on my first ram, a 2009, i had a power washer remove.a flake from the front bumber, that was in 2012 about... had that spot fix blend and clear coat done and from memory it cost me about 120$ back then thats cad so about 90$ ... im sure today it would be more expensive maybe the double or about... but just buying primer, the clear and having the correct paint done for you will bring you in the middle of that price about... so maybe worth it to bring it to a shop
having a spot repair will not be as.perfect as having the whole bumber done but in my case to be honest we could tell after... i guess it depends if the person that does it i skill in doing spot repairs...
bit for sure the rusted area has to be perfectly clean, sanded and cleaned with thinner or something similar
until you fix it you could put some grease on the rust to make the rust stop rusting