Getting the first layer height correct is another key with PETG. You generally don’t want to have your Z offset further down than it takes to get an even layer. Find that level, then adjust the extrusion multiplier on your filament in order to fill in any minor gaps. Lowering your Z offset to fill in those gaps is going to contribute to buildup on your nozzle. As far away from the build plate as possible has been the right level for me.
I can definitely print closer to the bed with other materials than I can with PETG.
They will limit, but not by as much as you’d want. Plastic bags still allow moisture to pass through even when they’re airtight. I live in a swamp, so drying is a necessity before every print.