TBH if this is a permanently fixed painted shelf I would maybe just put a bit of a chamfer on the ends and caulk it. I assume that that it’s not a huge angle so the gap will be not very big and chaulking is a proper job imo – you’ll never get a completely gap free joint without chaulking. You’d save yourself both time and money.

Mols: A 1000W router with a 1/4″ collet is only suitable for removing a tiny bit of material at a time; say a 5mm radius round over on something hard like ply.