How would one solve this tub-length issue?
First thing I'd do is challenge whether the wall finish stack-up you described is actually required.
Could furring strips or hat channel be embedded in the same plane as the XPS foam? If so there's no loss of dimension due to the furring strips. Are furring strips even needed? Could the subsequent layer of drywall or backer board be held by adhesive, or by screws through the foam into the block wall?
Why two layers of drywall or backer board? Would one layer suffice? What about a thinner alternative such as Schluter's KERDI membrane? (I've never used that product; it's just an example that caught my eye.)
Also: the water-shedding layer in the wall could (should?) go in front of the flange of the tub. When placed there you don't have to allow space for this layer in the rough-in sizing.
If you do go with the 54&quot; tub and need to make up 4 inches somewhere you're going to have to just build a box. It can be completely invisible if you make the box the entire width of the room, ie make the whole room 54&quot; wide. But maybe there's a way to avoid that and also make the space useful. For example, frame a built-in shower bench opposite from the faucet.
If the the 4&quot; build-out box extends wider than the width of the tub by one or two stud bays then that 4&quot; plus the 3.5&quot; depth of the existing wall could be turned into a built-in storage space. It's kind of shallow, but.. it could easily fit rolls of toilet paper, feminine supplies, cleaning products.. a whole lot of rubber ducks (Youtube, James Veitch Is A Terrible Roommate)..or hide-away towel racks, maybe?