Shower help - hot water / pressure

I’m in need of advice. In my master bathroom, the shower takes a really long time to get warm (not even hot anymore) - I had googled and it suggested sentiment buildup in the cartridge.

I went through a lot of struggles to replace the cartridge. First, I couldn’t for the life of me get the screw in the handle loose - I tried all the suggestions on here for a stripped Allen screw before I finally drilled the hole to get the handle off. Then, when trying to remove the old cartridge it broke in half and I spent a significant amount of time getting the broken piece out 🥴

I finally installed the new cartridge and new handle - only to still have the same issue. The water takes a long time to get warm, never gets hot and is low pressure- even with no shower head installed.

The second shower gets hot, and had excellent pressure (same shower head). The water in all sinks and even the tub in that bathroom gets hot - so I’m at a loss. Any suggestions as to what it might be?