There's a brand of spray paint called Krylon that's fairly hard to get in the UK which is supposed to be very good. Waiting on one of the few companies that stock it at a reasonable price to get it back in stock. Unfortunately Duracoat is almost impossible to find in the UK, and I couldn't find anything for Lawsons either.
I was thinking one way of doing it would be to just apply a normal primer, metallic layer, then a layer of matt black Krylon, with matt clear, then allowing it to wear naturally. As you said that would be boring, but I'm still a bit sceptical about model paints standing up to abrasion and salt / acidity. The issue is that there aren't really any suitable metallic spray paints. I don't know if there's any way of partially painting over that with matt black so as to not completely hide the metallic aspect, or to simply darken the metallic gunmetal paint, then put a matt clear coat over it.
I don't know if those would look any good. I may just have to test some methods out.
As for the cerakote, I had a quick look and it's apparently 120'C/250F. I baked my PETG filament in the oven at about 80'C to dry it out which caused the filament sticking out to sag and the filament to stick slightly together. The print temp is around 240'C, but I don't think it would be safe to heat it up for an extended time at 120'C and expect it to be the same shape.
Also for the sticky thing, are you sure it's not your sealer? If it doesn't set properly (sometimes chemical reaction), it can stay sticky. I haven't had any issues with my guns being sticky / tacky (I didn't use a clear coat, and it did wear through eventually).