From experience, my PEI sheet did not stay flat after multiple use, this may be causing the problems you are facing. It could be the adhesive is failing on different locations.
As you mention, a new sheet of PEI may work for you; indeed the glass will not be necessary then, glueing it directly onto the aluminium bed would be enough provided that the metal bed is straight, glass is straight as a result of the production process to make glass.
Note that printing PETG filament is not recommended without glue stick or any other bonding agent like hairspray. This is not to get it to stick, but to get the print loose from the build plate, PETG fuses to PEI...