I find typing the comment out, then deleting it and not submitting kinda gives my brain the 90% feeling of needing to say something. Only place I don't have to do this is 4chan, where the worst thing to happen is that your comment gets ignored or something mean is said to me (oh dear!)
I unironically just closed this tab before submitting out of habit and reopened it to submit this
I've found increasingly I'll submit a comment only to go back 30 seconds later and delete it.
I participate in a number of 'old school' forums, never anything like reddit or discord. On those forums, while I have posted on some a fair amount, I actually find that most of the time I spend 15 minutes writing up a post, then delete it. There are a number of reasons I don't hit the submit button. Sometimes its because I see that a lot of other posters will disagree with it, and I don't think they will argue rationally and in good faith; but the most valuable posts I don't submit are when I get to a point in my argument when I realize that I'm wrong or that my opinion or point of view is badly supported or any numer of other things that force me to re-evaluate position. I've probably held that position for a while thinking I'm right, but actually formulating the argument forces me to confront my biases or mistakes.