They sit 3mm proud. This is not an issue if they are hidden, me thinks. However, you should be able to not use the support ring if it is an issue. (I’ll modify a design today in attempt to reduce the amount it sticks out.)

Also, these are intended to be extremely secure and almost permanent. If you want it flush, you can probably pound it in a little more with a hammer. If not, leave it proud and it gives you a little meat to pull it out of the wall later.