6GB will be fine but obviously 8GB would be better if you want to open many apps simultaneously. But since you mentioned using Waydroid extensively, then I would recommend the 8GB. I've used 6GB and 8GB variants of the FP4 and Waydroid can be slow on the 6GB variant when you have other heavy apps open like a browses or a webapps. Remember, you're running another OS which would use more memory.