Creality bases are notoriously NOT flat although I’ve heard their quality has improved over the years. I have one of the 1st-gen Ender 3 Pro machines with a terrible dip in the aluminum base. One nice thing about a thicker bed material is that you can use discs of aluminum foil the shim the base. I started with a glass bed and 13 layers of foil to get the glass reasonably flat. I have since moved to 3mm G10 with a PEI sticker which is working pretty well (in case you want to try PEI again). I found some scrap G10 material on ebay and chopped it with a table saw, sanded all the edges, then got a PEI sheet that was 10mm wider than my bed to allow for some slop. Putting down the stickers is a lot easier if you have someone helping you. Then trim the overhang and you’re ready to go.