The protection needs of a MBT and a SPA are too much different for a standardized armor scheme to make sense.
With 1950s and 1960s technology, yes; but not with 1970s and onward tank technology.
Essentially, tanks have become Rolled Homogenous Armor enclosures for slabs of fairly thick secret armor inserts, which can be swapped out to improve protection when needed.
In fact, in order to get maximum protection from most modern composite and ceramic armor systems, you need a dense cover and backing for the ceramic/composite, because the backing and cover plate actually help increase the resistance of the plate to KE by 25% over what it should be.
So if you take the armor insert technology to it's logical end, modular armor; it opens up new design approaches:
What if you simply design a common 40 ton tracked vehicle chassis; and then mix and match armor to each role?
The tank can have the 9 ton 600mm thick ceramic/composite chobham frontal armor insert; while in the SP Artillery piece, the 600mm thick insert space in the frontal armor are is left empty, and the weight is instead allocated towards:
5 tons of 25mm SHS Steel Armor plates bolted onto the top armor for artillery fragment protection; followed with 4 tons of ERA bolted on top of the SHS plates to protect against DPICM.