The reasons to explain your situations can be:
1) plumbing valves/check valves
2) filter that is clogged from algae
3) debris on the pressure side of plumbing
This is if I assume that indeed you observed lower pressure when the system was initially installed. I'm wondering if the gauge was actually "lying" to you on the low side and showing you say 12 PSI initially because it was clogged from some PVC grindings from installation. And maybe after it got cleared it actually showing the correct pressure. Maybe your pump+plumbing is such that you have a 25+ psi.
I think from my experience, it is not good to operate system at close to 30 PSI. It most likely means you are wasting energy and not efficient. that is if you even get a decent flow rate.
1) Now, from the picture, i would look into the salt cell....make sure it is clean....Then, what is that piece after the cell? is it just a temperature probe port??
The heater, does it have a bypass valve? Can you tell if it is fully on either position?
Also noticed you have a water valve on top of the pump. You can get a $10 pressure gauge from home depot that plugs in directly on it. It is worth to get it and have a sanity check...just to make sure.
2) since you opened and verified that the filter media is clean, i don't think it is your issue. I had exact issue because of green water that clogged a DE filter medias just couple hours of operation...
3)check pool returns maybe you have those rotating eyes that can be closed or something.
I would also try to select only one of the intakes to the pump like and bypass the booster pump. then change the valve orientations and compare numbers.