good pressure in tank, low in pool

[YanJin]

I have a pretty old pool, anothony pools, apollo model. Its vertical tank that for one im not sure if it needs DE, it has a filter in it, its not sand. I replaced the pump some years ago with a speck pump and it worked fine for a while
over the last coupld years its gotten more and more difficult to keep pressure up. this year i managed to fix that problem but the other issue is that there is not as much suction from the skimmer, even though the pressure is right on the money at 20 PSI. also there is little to nothing coming out of the vents.

i have checked the pump part of the pump (not sure of the technical name) and it spins free and debris free. the filter is not overly dirty, and i used the plastic bag trick so see that there is nothing stuck in the pipe leading from the skimmer.

The kicker is that after i open the pump or filter, the system pulls water in on its own, and very cleary and powerfully pushes all the air out of the system and out of the vents, resulting is powerful bubbles.

But within a couple minutes it slows to near nothing, despite keeping the same tank pressure. the pressure relief valve on the filter sprays water some 4 ft (vertical tank) yet no power in the pool.

any ideas? and should i use DE?