I'm VERY new to Reddit, but I'm familiar with your issue. You are right to feel angry when you're trying to share something to others and it gets rejected. All I've done is post a cover and a Linktree, and a couple of snippets for a preview. But I posted in webcomics, so there may be a different audience there.
Yes, you are right about that one-page-out-of-context thing, and that's not a bad marketing strategy. I do it in the Tapas forums. I think that Redditor is just toying with you, but I don't know. Maybe try a different subreddit? Or create your own?
I don't know that many, but you might have to do some digging, unless others here are on others that they can share...
r/webcomics
r/webtoons
r/tapas (a guess, but it should exist)
Best to experiment, and don't let that toxic bs get to you, I like your comic, and subbed yesterday(humble brag). Just keep trying, you'll find a place. Mine is definitely here atm.
Godspeed!
PS, maybe someone should make a longformComic subreddit? post en masse every few weeks? IDK