Yep, am aware how the team is doing a good job, and faced the problem of balancing stuff myself when i first started doing balancing for my version of BDA. Pretty hard testing and retesting until it seems balamced then noticing one aspect of the weapon is imbalanced.
I do have knowledge of hoe AP and HE works, have been in wargames and studying (although only on hobby level) armored and aerial warfare. I do understand that APHE rounds are seldom used these days and it's mostly pure APDS solid rods or pure HE or HEAT.
Problem with the current armor/HP system is that HP and armor can both kill a part, and HP degradation is independent of armor. The armor depletion works good as far as I'm concerned, but problem is that you can just spam low AP high HE rounds (ex 20mm) and destroy a thick armor plate with something it would be mostly immune to irl by draining the HP, something the M230 and goalkeeper rely on. It's kind of like a WoT/WoWP style of damage and not some simili War Thunder or Wargame Red Dragon type of HP damage I'm trying to emulate. In WG:RD heavy AP rounds can damage heavy armor, but small AP and small HE rounds (ex a 30mm M230) can also damage armor, but only by tiny increments. You'd need to fire for minutes at a T-80BV's rear armor with a Apache's gun to entirely kill it. In BDA with turret weakness (while irl tanks' turrets are most often their most well armored part to protect crews) a tank is quickly rendered useless with such a gun, sometimes even before the armor itself pops off.
My take on it was to consider HE damage on armor by small bullets and autocannons (sub 30mm) to be negligible on armor but significant on lighter armored vehicles. AP would cause damage to heavier armored vehicles, and heavy HE shells would fall out of the context since they're most usually used against infantry in a modern context. Thus I'd sacrifice a middle ground between heavy and light vehicles, and create an AP/HE balance more around light maneuverable wheeled vehicles or planes and heavy slow vehicles.
Unarmored vehicles would be vulnerable to all including 50 cals, light armor would be vulnerable to anything bigger than 2 AP and something like say 10 HE, heavy armor would shrug off most HE and suffer only from AP. Armor wear would be assumed to be negligible when coming to the last category.
As for pure APDS (high AP 0 HE) shells also doing lots of damage to light armored vehicle, ingame it would be reasoned by spalling damage, something which makes pure solid AP shells viable irl and not just go through tanks like butter and do no damage.
ATGMs would have a high enough HE armor to destroy both and act as HEAT round which works good on generally all kinds of armor, and justify craft destruction by assuming spall and crew placement absorb the damage (even if there is none)
It would have disadvantages compared to current BDA, most notably perhaps removing the branch of medium tanks from the balance, and the inability to portray heavier HE rounds that can also hurt big armor, but I'm aiming for a balance style more towards advantages and disadvantages for most building styles.
If you need help testing I'm always available. I've been modifying BDA and balancing it for a discord RP I participate in, and building lots of vehicles for the last year (although very few are posted on the forums). You can ask @Spartwoor @THIRTY9CLUES if you want some history of me, and I also reccoment them both for play testing as they have extensive experience. I've been wanting to rebalance BDA to make it more realistic for a while, and I'd be happy to help make it happen