Autodesk EAGLE Routing of STM32WLE with tiny UFBGA73 package
Not being able to use a package due to manufacturing constraints is a real and frequent thing.
Solder mask expansion has a real role to play - it gives tolerance against offsets in manufacturing: 
So obviously you don't wan to shrink the opening too much... This is really something that's driven both by the device datasheet (less) AND manufacturer requirements (more). PcbWay has a blog post which seems relevant and they tell you that you can go down to 50 microns of expansion. Also note that ST says 330 microns typical, depends on solder mask registration tolerance, by which they mean to stick with what your manufacturer can deal with: