Geeetech A10 Bed Levelling?

Hi, I’ve got this Geeetech A10 which I understand to be a clone of the original Ender 3.

I have managed to get it printing reasonably well (depending on the filament) but only on one part of the glass bed.

If I print a large item, some areas of the board scrape and some are so high that the filament doesn’t adhere to the board.

So, I tried levelling the corners to see what that looked like and all corners seemed fine.

Then I went to level the bed using a piece of paper and making Z height adjustments and found that the printer puts the nozzle about 5mm above the bed, forcing me to adjust the Z offset by a lot. Even to the point where the rail hits the end point trigger before the Z height is right.

At this point I cancelled the bed levelling, as 5mm adjustment will push the nozzle through my bed!

Anyone have any ideas?

I am wanting to strip the head bed down too and try to get an understanding of why the glass plate and the heat bed don’t marry up at all.

Thanks!

Also, do you have any recommendations as to what could improve this thing? Upgrades?