As of now, I feel like the distribution of Flash is somewhat lacking, possibly because it lost its HM status as time went on. This is fine in the mainline games, where its not very useful in battle, but flash is very important in PMU - in fact, I would argue that flash is a necessity in dark, small-room dungeons.
According to bulbapedia, there isn’t a single pokemon past gen 6 that can learn flash. I’m not saying this is a bad distribution, but in my opinion, it should be even better; an HM move (or something that fulfills a similar function, like flash) should be able to fit on almost any team. If you had to ask me for a simple fix, I’d say that anything that can learn dazzling gleam should also be able to learn flash. That makes logical sense, right?
The exact same argument could be made for HM Rock Climb, which is a necessity to access some portions of the overworld. It, too, is a former HM which I’d argue needs better distribution - aside from the Lycanroc family, there’s nothing past gen 5 that can learn it. Maybe give it to more rock, fighting and steel types?