The thing with the countersunk rivet also is after time, they do work loose and move so several things happen, the countersunk hole breaks away from the rest of the panel or as the rivet is made of a harder material, it will slot out the hole it goes through or as it weakens the surrounding area, one ends with rusty, split panel with an insecure fender support tapping away at the underneath of the panel. So, what you have done is exactly how I would of tackled the problem, just make sure that the nuts on the inside of the fender brace are locked tight. Also I would suggest a plastic or rubber sleeve placed on the thread exposed to protect the thread for any future maintenance and ease of removing the nuts.