The forums are the place to do it, generally. There's also an in game "Suggestions" button as you note, but, that obviously won't give the devs a meter on how popular an idea might be (or might not be), doesn't give a chance for spinoff suggestions in discussion, and so on.

The devs have demonstrated they do read the forums. Certain things they'll respond very quickly to.

The issue with healers is, simply, the devs are not on board with many of the sentiments here. And since the communication is almost always one way, we don't really know what they're thinking or what compromises they might be open to, if any - other than what we can infer from the directions they take with patches which sometimes align with suggestions on specific abilities (eg Misery, Energy Drain, Neutral Sect) but almost never on overall direction. So, here we are, essentially deadlocked.

A secondary/contributory issue, is, I think: Healers are almost never talked about in the media, both in formal interviews with publications and by youtuber/twitch types. Now, with the lead up to Endwalker we did see a bit of change there - compared to the usual total lack of discussion on it at all, there were a few questions and some small segments in influencers' videos occasionally. But it's still very underrepresented - no one of import/influence really wants to spend much, if any, time discussing healers. So... here we are.

This is the best way, unless you have notable influence on social media platforms or the press perhaps. But SE holds the reins and disagrees with much here, and even using the best avenues won't get around that.