I would use a relay, not a speed controller. I’m not sure how the ring light would respond to the speed controller waveform, but there’s no need to find out!
When we’ve used that style of ring light in the past, we’ve powered off the VRM without any issues, just left it running full time. HOWEVER, be aware that if the ring light shorts out, it’ll take the VRM with it. If it’s the same VRM that’s powering your radio, that’s very bad. So we would use a second VRM.
Since then, we’ve found a different style of ring light (from supper bright LEDs, I believe). It’s completely enclosed, so there’s nothing that can short, and it comes with its own in-line voltage regulator, so we can put it on a breaker on the PDP without needing another VRM. very nice, clean package!