I had the same problem with my CR-10. The issue was that the weak original bed springs were too weak to hold the bed. The vibrations of the printer allowed it to move, causing the first layer to not adhere very well. I also had filaments get moisture after a week or two.

When people say upgrade the Creality bed springs, they aren’t kidding. PLA is forgiving enough on the first layer, but upgrading those bedsprings was necessary for me to print PETG. I made them stiffer by adding 5-6x washers on each post to pre-compress the springs. This makes them much stiffer and stay put.

I also bought a filament dryer. I put a new roll of PETG in the dryer and put it in continuous mode at 55°, and print PETG directly from the dryer.

I still have to print PETG pretty slowly compared to PLA or ASA (my favorite material).

To OP’s original question: I do a full tuning of PETG with a new maker/roll. PETG is a co-polymer. Each manufacturer could have a different amount of glycol on the PET chain, or even a different distribution (e.g. ABABABAB copolymers behave differently than AAAABBBB copolymers even though both are 50% B). Colorants can also affect behavior.

PLA I really don’t tune. I add 5-10 degrees when I’m printing quickly. That’s about it. I recently was printing a spacer out of the last bit of a PLA+ roll I had. The filament ran out and the printer halted. I loaded a matte PLA to finish (I slowed the printer down and bumped the temp up manually by 5 degrees since this matte stuff has layer adhesion issues), and it also ran out before the print was done. So I loaded a roll of transparent PLA that I bought 9-10 years ago (and was stored in an attic for 2 years) and is very brittle, and it printed just fine. The print looks strange (tan to matte terra cotta to transparent purple) but works great.