Hi there! I can't answer all your questions because I'm new at this myself, but I can answer a few.
Once a week is the most common. Twice a week also seems to work well for a lot of creators. I think more than that may result in your audience becoming fatigued about the constant notifications.
Perhaps you can consider posting two pages in each update, rather than one? I'm doing this with a few upcoming posts, as I have a couple of pages which are full-page illustrations/establishing shots, and too bare-bones to be posted in isolation. This will go down better on WEBTOON in particular, where the audience favours longer updates.
Promote, promote, promote! Reddit has worked well for me, as does Twitter. Watch out for ratings trolls on both sites, though. It's a common problem.
WEBTOON and Tapas have the biggest audience, by a large margin. So no, not really.
Unfortunately, yes. Especially on WEBTOON. I create my comic in page format too, because I also want to print it. But I now structure the layer groupings in such a way that I can easily switch entire panels on or off, which makes adapting it to a vertical format quick and easy.
I use Tapas to host my page-format version, and WEBTOON to host the vertical one. The audience on Tapas is smaller anyway, so using a less-popular format here doesn't bother me. WEBTOON, though... It has more people reading on mobile devices, and a far bigger audience, so that's the site I'm more ambitious with.
