There are various chemicals that are supposed to infiltrate, penetrate, or whatever different pieces of whatever that are stuck together.

The first thing I'd do, really, is to look at the materials and check via google for hints. Is it cast aluminum? Cast iron? Is the pump made of the same material as the block? I can't offer any hints tailored to these particular materials except for you to take note of the materials and look up what you can.

Case in point: decades ago I was asked if, since I was such a smartass, I could figure out how to remove a shower head in my girlfriend's parents' house. They wanted a new one. Two plumbers had not been able to remove the old one.

So I thought of "difference in temperature."

I actually got into the shower and turned it on, which the plumbers guaranteed did not do... I did something nuts like turn HOT on full, then put an ice cube up against the nozzle part. Whatever the details, the shower head came off easily with two pairs of pliers.

Also be sure that you really have removed all the bolts. The model you mention (ahem) might have some other thing that holds it in place.