All BR changes in my suggestion are for Realistic Battles.
USA
-10.7 M1 Abrams (and M1 KVT) needs M833 ammo so it can more effectively fight much more armored vehicles at 11.7 in full up-tiers. A tank being forced to fight 11.7 opposition tanks should not be fighting them with ammo from BR 9.0. The M1 under these current conditions simply cannot effectively compete.
-XM1 (GM) and XM1 (C) should both have access to M774 and both have the BR of 9.3.
Currently the XM1 (C) still sits at 9.0.
When the XM1’s were first added M735 had ~350mm of armor penetration and M774 is not much stronger in performance at 372.
From a gameplay and balance standpoint other tanks at 9.3 have similar mobility performance and have access to much better ammo such as DM33 and DM63.
From a technological progression standpoint it also does not make much sense to have the older M60A3 use M774 and the more Advanced M1 use M774, but the tank between them only has M735. For players who purchased the tank originally with the old M735 performance, this would be a consumer friendly move as it would restore the quality of their original purchase.
M60A3TTS (US)
-American M60A3 TTS should have access to M833 ammo. Other M60’s in the same BR have DM33 which far exceeds M833 performance.
Example China’s M60A3 TTS has DM33 at 9.0.
XM8
-XM8 Light tank should have access to M833 or even M900 as it historically could fire both. Adjust BR to 10.0 if given M833, 10.7 if given M900.
Additionally it makes no sense that the previous iteration of the XM8, the CCVL, would have M833 ammo and the later design would only have access to older M774 ammo, if anything they should be in reverse with the CCVL having M774 with the XM8 having M833.
TCM AGS
-The AGS at 12.0 should have M900. It’s ammo was not improved despite receiving a BR increase where it now has to fight the most heavily armored vehicles in the game with only M833, a round with only ~395mm of maximum armor penetration.
HSTVL-RDF/LT
-Lower the BR of the HSTVL and RDF/LT to 11.0, or give it it’s proper Delta 6 APFSDS ammo to fight 12.7 tanks if the BR remains 12.0. Functionally it is very similar to the 2S38, which has a much lower BR and is actually more effective in most cases as it has much more ammo and a faster fire rate.
M10 Booker
-Lower the BR of the M10 Booker to 10.3. It has effectively the same combat effectiveness as the M1128 which is at 10.3 with M900. The booker has no autoloader or low profile turret and is larger than an M1 Abrams giving it horrible survivability given that both crew and ammo are much more exposed relative to the M1128. Its only real advantage is that it is a tracked vehicle with a tighter turn radius than the M1128 giving it better cross country mobility.
M4/T26
-Lower the BR of the M4/T26 to 5.7 it is effectively a closed top M36 Jackson and that hardly warrants a BR increase to 6.0 given how weak the hull armor is and the mobillity is the same as any previous sherman. It also has a higher medium tank spawn point requirement to balance this out compared to the 5.3 M36 Tank Destroyer.
T20
-Lower the BR of the T20 to 6.0. The tank is hardly adequate at fighting 7.3 tanks in a max up-tier which is the norm.
Russia
T-80UD
-increase the BR of the T-80UD to 10.7, Armor wise it is strong.
BMPT-72/BMPT
-Increase the BR of the BMPT-72 and BMPT Terminators to 11.7. They still win most fights in their BR range and especially below.