My rear brakes are definitely the brembo (mid Jan 2023), I would prefer the Mandos TBH but you're not going to see this change any time soon. They were never monoblock- they were sliding caliper brembos done up to aesthetically match the front fixed calipers. Maybe with Highland we'll see Tesla do one brake system for all on the 3, but not today.
You're not going to see this kind of cost reduction any time soon, reason it happened on the Y is that the front and rear brake diameters (and indeed the front calipers) were always identical between LR and P. The LR setup on the Y was already just as good as the P setup for almost everything, P rotors have slightly more thermal mass on the Y but they are not the steel/iron composite the 3 has, and they never were. 3P is still a fair bit uprated from the 3LR.