The learning curve for SPAA is extremely steep (Especially for single-gun ones like 40mm), and for what they do they give poor rewards. Could theoretically everyone do this? Yes. Is it ever practical? No.
The overall solutions I see for the CAS problem are fairly simple:
1: Allow scouting of aircraft by SPAA, and give a short-range lead marker to scouted aircraft for SPAA and HE-VT/HE-TF. (Yes, this includes flak guns and howitzers with those rounds, though only when loaded.)
2: Give pantsir-equivalent SPAA to all countries, and fine-tune the balance of AGMs at top tier. As an addendum to this, give score for shooting down missiles/bombs.
3: Give HE-TF the ability to act like HE-VT on scouted targets only.
4: Variable SP cost for SPAAs based on the ammunition carried - i.e., ZSU-57-2 with APHE would be the same as a medium tank, wheras with only HE it would be the same as current.
5: Y’all need to accept that SPAAs will need to ro AT work, and be fairly decent at it. As such, nonsensical limits on APDS should be removed unless it is specifically historical (Gepard and Gepard-likes). APDS should also be provided to 40mm SPAA that need it, (Lvkv 42), but with the SP cost listed above. HE-VT for 40mm should also be added back to the VEAK, and it should be given APDS. (Being moved up afterwards, of course.)
6: Just general addition of more SPAA to flesh out the tree, like the Finnish ZSU-57-2M (Finnish ZSU-57-2 with HE-VT and radar.) Should be done at all BRs.
And finally, giving all SPAA two stock belts - current default, which generally consists of half AP, half HE, and a full HE belt. For SPAA with HE-VT, they should get a half HE, half HE-VT belt.