Congrats on the progress.
20 pumps will just get you to the point of checking that the piston does indeed move now, but it won't be sufficient for spearing. Not even close. I would start at around 15 bar for your first outing as you should be able to load that. According the table Pete attached, that's 130 pump strokes.
As for being "pumped beyond full" it's true in the sense that it was pumped to a degree where you couldn't load it on land (loading on land is never a great idea any how). As Pete mentioned, it takes a very good push to load these when they are at the higher pressures needed for spearing. Bear in mind, that band guns are not easy to load either and the energy you put into the loading effort is pretty much the power you get out of it in terms of shot power. So, you actually want it to be hard to load. If it is easy to load, it means you wont have a powerful shot.
All of us pretty much run the pressures as high as we can load the gun without tiring ourselves out. A lot of it is just about getting used to how to load it, so I think if you start at 120-130 pump strokes, you should hopefully still be able to load it as a beginner.
And please do not load it on land. If you do, only do it at lower pressures to check that the trigger works and do not shoot the shaft out, but rather jam the speartip against something and "shoot" as you lean onto the gun but slowly moves it backwards. Hard to explain, but the idea is to let the shaft out gently. If you shoot it on land, with the reduced friction (no water) you might bust the piston or other internal parts. And it is potentially dangerous no matter what.
I should say, you may risk having a leak just by having taken the air out. I know it sounds weird, but seals (o-rings) which have been sitting on pressure for as long as yours have, might have deformed. You will see if that's the case once you take it up to pressure and shoot it a few times. Actually, already now, you could stick it in a sink and look for bubbles.
Also, when you emptied the air out, would you say the mist was a bit oily? There should be a bit of oil in there, so it. would be good if you feel it in the air.
Finally, I would personally take it apart now and service it completely. On the other hand, lots of spearos subscribe to the method of "use until destruction" when using these guns, so since you haven't taken one apart yet, I don't see why you can't just try to use it and then fix it/service it later on if the need should arise.
The gun looks complete but as Pete noted, you need a loader.