"Enough to retain structural integrity for the rest of the tank that isn't (N)ERA, with frontal arc also functioning as a backstop for HEAT's remaining penetration"
The "excess steel armor" problem came around due to increasingly powerful HEAT shells and ATGMs, their relative inaccuracy asides. If one of those connected, they would probably go through a lot more steel than you could reasonably fit on a tank.
The Germans and French responded with the Leopard 1 and AMX-30, which were intended to trade armor for speed to get out of the line of fire with. The British had Chieftain, which relied on sloping to hopefully deflect was fired at them (this didn't work, hence Stilbrew). The Americans had the M60, which was designed for fused silica that would probably be pretty effective against period HEAT, but was never built with it.
Leo 1 and the AMX-30 don't have much of an excess steel problem, but they don't have the growth space for significant upgrades due to how light they are. Chieftain... well, eventually a version of the design shorn of most steel armor and replaced with Chobham, and with hydropneumatic suspension became Challenger. M60s soldiered on for far longer than they should have in US service no thanks to MBT-70. Marine Corps M60A1s eventually got an ERA kit (apparently not Blazer) in time for Desert Storm, but remaining stocks were rapidly sold off afterwards. Israeli M60s (Magachs) got very extensive (N)ERA upgrades following the agonizing experiences of Yom Kippur. Sabra is a further evolution of late Magach design principles, but it's really close to the absolute maximum you can do with an M60, assuming it's not already there.
You may have noticed that I didn't mention Soviet tanks. Starting from the T-64 on, all of them had NERA protection on the frontal arc. BDD kits with similar (if not identical) working principles to Chobham were made available for T-54/55s and T-62s as well, although never for export.
EDIT: General Dynamics actually did propose your suggestion of putting a new turret on an old tank with the M60-2000, an M60 hull with an M1A1 turret. Nobody bought it.
Example of a turret with fighting compartment that were made to upgrade T-72s and T-80s in Russian stocks:
New turret had modular frontal armor, allowed to quick replacement if damaged and could be upgraded easily with new packages.
Burlak turret also had a bustle, space inside of it was used to mount second autoloader and remove number of shells outside of original autoloader from tanks hulls into separated from crew compartment.
On top of that turret also was equipped with APS to protect sides and RCWS that original T-72s don't have. Never saw service.
Is there any way Sherman tanks could be upgraded at reasonable cost to still have a role on the battlefield? Assume that your military will never fight with a world-class army (U.S., Russia, China, etc.) and instead will only fight with second-rate armies using 1990s technology at best, or with terrorists, or go on peacekeeping missions.
I'm thinking that the Shermans could have their turrets removed and modern autoloading turrets from other armored vehicles--like the 105mm Stryker gun, or the 40mm autocannon from the CV10, or the 30mm from the BMP-2 (would any of these fit in the Sherman's turret ring?)--could be dropped in, along with their sensors and computers. ATGM launchers could be installed as well. Explosive reactive armor bricks could be attached to the outside, as is common among modern Russian tanks.
Assume that your Sherman fleet is 1,000 tanks, in various states of (dis)repair, so you have enough spare parts to last for many years.