The PIAT doesn’t need a nerf as it is already inferior to the Panzerfaust in just about all ways apart from AoE, hence why it is more often used as an Anti-Infantry weapon than an Anti-Tank weapon.
Not all the of shells should have a larger AoE, stuff without an explosive filler and HEAT ammunition didn’t really have a large AoE either due to the lack of explosives or the explosion being directed, they should have a higher velocity/less drag or impact damage than explosives shells to compensate.
The fact that stuff like HESH doesn’t damage infantry with its AoE is quite silly btw, even if it is less effective than HE due to the lack of fragmentation…
The AA guns and auto-cannons (20mm and 30mm) shouldn’t have a large AoE either, especially not if you are using AP instead of HE rounds.
Note, this doesn’t me the regular HE other explosive shells shouldn’t have an actual AoE, because currently they have almost none, and especially the 90mm howitzer on the Churchill could need a bit more power as it can’t really do anything else (even though it might be the only cannon with actual AoE).
But I generally agree with your points ^^