PETG is sticky stuff. It loves to stick to itself. So using supports printed in PETG on a PETG print means its going to stick to itself very well. I haven't found any way around it. Normally I use whatever my layer height is as the offset from part. They come off. Not easy but they do.

As for not sticking, CLEAN your print bed with acetone just prior to printing (while its hot). I have printed things similar to yours without a flat base. I have a paper towel that has remnants of ABS slurry on it. I wet it with acetone and wipe the hot bed with it. This applies a THIN layer of ABS on the PEI bed. PETG sticks to the ABS like glue. I have never had one pull lose using that method.