No amount of fans will help, the A18 is just way too small.I was thinking about the 3800X but that is a 105TDP which seems to exceed both the above mentioned OEM heatsink and the A18.
I wonder if adding some high speed 40mm Fans would help a bit however I would like to avoid any thermal throttling.
You should definitely go for the L3, but if that isn't an option, you can try mounting a non-AM4 cooler like I did.