If you're looking for some soft feedback and a boost to get yourself going, posting a Tapas novel would be really good. It will at least show if some people are interested, even if the audience for comics and novels don't always overlap. On the other hand, if you're looking for Critique with a capital C, you might be better off posting here on the forum or to a writing group. In my experience Tapas commenters are happy to tell you a few things they liked, but are less likely to give honest critique to help you improve.
Webtoon on the other hand can be very blunt about what they don't like, but that's not always helpful either
But if you have an idea that you honestly want to pursue as a comic, I don't recommend writing it as a novel first. Right now I'm writing for both a comic and a novel set in the same canon, and let me tell you it has TRULY opened my eyes to how different the two mediums are. The visual nature of comics allows for very different opportunities with setting and pacing especially that would be totally lost if I had written it as a novel first, and there are things I've written in the novel that I would have no idea how to portray visually without digging into clumsy flashbacks or inner monologue.
True there are plenty of comics on here that have been adapted from novels, and I'm not knocking them at all, but if you're not writing with your medium in mind, I feel like you're missing out on some of the best things about comics.
I recommend writing your comic ideas out in script form rather than prose. Let yourself think about the structure of the page along with the actual writing. You can't really post that as a Tapas novel, but you could still solicit crit from us around here on the forum.