I have never had to use a glue stick for PETG on textured PEI.
I also have never had warping issues with PETG on PEI.
The trick to PETG adhesion on PEI is to wipe with 70% or higher Isopropyl alcohol (IPA) in-between print jobs.
I’ve actually ran tests on this:
If you are cleaning with IPA and are still having issues, make sure the part cooling fan is turned off.
Most PETG will not bond correctly if it cools too fast. There are rare instances with the part being too small that the fan has to cool the layers, but 90% of the time there should be no part fan cooling.