multiple questions here, so a break down:
To Webtoons or not to Webtoons?
Generally speaking there's not a downside to posting several places, besides time spent. For example I post to both Tapas and Webtoons because saving the 800px wide pages for Webtoons doesn't take much time so I don't mind too much. Time is a real cost though, so keep that in mind if you start posting a bunch of places. For example, with my current comic if I were to drop one platform or the other because it was taking too much time, I'd drop Webtoons currently, just because my comic performs much better on Tapas.
Promotion?
This thread is a pretty good resource:
My own weekly routine is written there as the 4th or 5th reply. The tl;dr version is: posts to social media (fb/twitter/instagram/reddit) & promotion on the forum (mostly the "newest page Jan 2019!" thread). Realistically unless you get the website to promote you themselves (either through being a premium creator, or things like the spotlights on Tapas) not a lot of readers will find you on their own, so you have to do the promotion legwork yourself. This is another time cost (I spend like an hour and a half each sunday doing this).
Creative Promotion?
One thing I do is take my thumbnails that I use for the episodes, and edit them into little "Update!" images like this:
idk if it really has a big effect on my promotion (vs. just posting the link and using the auto-preview image) but I think it's nicer and doesn't take much time to do.
Also I commissioned this cool video trailer to show off my comic @u@