The only way to even begin “balancing” the Maus (and by extension heavy armor in general for late-game) is removing all the old long-since-redundant postpen nerfs for APCR, APDS, HEAT, HEATFS, and HESH, as well as further buffing large HE. APCR also needs a calculator overhaul to be much closer to real penetration numbers (like APDS now generally is).
Being a damage sponge is not fun if you’re just sitting there soaking penetrating rounds repeatedly disabling your breech(es), barrel(s), tracks, or all three. That’s just delaying the inevitable - it’s only fun when you can shoot back and not be barreled before you can properly aim and fire.
The artificial survivability imparted to the Maus (and late-game heavies in general) by ammunition meant to kill them having piss-poor postpen is why so many of them still sit at ludicrous BRs.
APCR and APDS with enough pen to comfortably punch through the Maus’s turret face should take no more than 3 shots tops. Artillery caliber HE already one-taps it. HEAT & HEATFS should be as effective as the first two, if not more so. The Barn and AVRE should annihilate anything they so much as hit unless it has ERA, period.
With the “fake survivability” gone from the Maus and all other late game heavies, their BRs would plummet. Then they’d be able to frontally stop shots more regularly.