Please explain to me how to succesfully print PETG on a PEI build plate.

[airscapes]

Ok, so cleaning filament is just that. When you remove the pla you heat the nozzle with the nozzle way above the bed and you extrude 200 or 300 mm. Then you remove it and insert whatever you are moving to, in this case PETG. Run 300-400 mm waiting 30 seconds between extrudes so the cleaner will get removed. If you do this the nozzle will never clog.

I would set your bed to 80C normally 70 is fine but you are having an issue.
I am going to attach a file if it will let me. This is a 2 walled square that is 3mm tall with wall thinness of .4mm.
Print this in pla using a layer height that will cleanly divide into 3mm. With that size nozzle and the issues you are having I would suggest you use .3mm layers.
Once printed measure the height of the walls, should be 3mm+-.02. Then measure the thickness of the walls, should be .8+.04 or so.. If the walls are 3mm tall but under .8mm you are under extruded.. if the wall is shorter or taller you need to fix the Z offset before the thickness can mean anything.