Much good advice up above.

Personally, I'd be holding back on piston lightening as described for a while......

Lightening the piston will give faster piston acceleration and may result in slightly less primary recoil, but COULD result in more piston bounce (surge) which is much more detrimental to hold sensitivity. The piston on this 25mm bore gun won't be that heavy anyway.

For now, I'd be paying more attention to technique and "get into" the art of springer shooting. This will generally mean employing a looser "hold", letting the gun move around, the whole point being repeatability of hold. If you grip a springer tightly, you cannot guarantee gripping it to the same degree each time.

Also, although you are using high quality pellets, there is no guarantee of absolute accuracy. This was shockingly demonstrated last week at The Bash....HW95 with high quality, JSB Exact 8.44 pellets. Rested. Range approx 15 yards and the groups were about 2" diameter! Yes, really! Before the owner explored other (much more complicated) avenues, fingers crossed I made a bit of a "stab in the dark" suggestion that he tried some other pellets. He had a selection with him. Switching to the same pellet, but a different batch number, group sizes shrank to just slightly larger than single pellet hole. Not sure if it was just a bad batch or just a case of this barrel not liking that particular batch.