The DRDT-2 has a documented limit of 0.040. When I dimpled at that thickness I believe I also did slightly additional debur to shave enough for flushness when I drop in a rivet.
Beyond this I believe you would machine countersink.
I don't recall if I dimpled or countersunk. I tend to dimple whenever possible. There's plenty of info on the forums about thicknesses that are appropriate to dimple and those which should be countersunk.
I learned some very valuable tricks during my flap construction. I've taken the liberty of sharing them with you in the following link.
Counter sink them. If you dimple you will still have to do some counter sinking because the spar is thick enough that you won't get the skin to nestle in the dimple properly.
When building, remember that hundreds / thousands of RV's have been built as described in the plans.