100mm for IFV is not a bad choice.
25mm/30mm cannon doesn't provide much for HE
Well a 30mm with AP rounds will cause damage to running system (i.e. wheels and tracks) and the optics from the front and side arc. From the side and rear you have a chance of hitting the ammo rack and taking out the engine.
If you have a light vehicle you can maneuver around the target into advantageous positions and hit turret rings, exposed systems, side, and rear armor. This is generally enough to do in most older soviet armor, AMX-30, Leopard I and defiantly APC’s and soft targets.