Yusss, I tend to fiddle with genres pretty often, lol.
On Tapas, I started to post under Fantasy, but quickly switched to Mystery as soon as I figured out just how oversaturated the Fantasy genre was
at some point I switched to Horror because I felt like Mystery was a bit TOO dead in comparison... but eventually decided to switch back to Mystery because it's easier to get seen there 
With Webtoon, I originally started to post under Supernatural/Horror. However, while Supernatural is indeed smaller than Fantasy or Romance, I noticed that I was still competing against some pretty big comics. The "smallest" one in the Supernatural Top 30 had like... 3k subs. So I went with Horror as main genre and Supernatural as secondary. Gotta admit that in my case ending up with a red banner was quite painful, because my thumbnail is all purple (which was PERFECT for the Supernatural category) and now it clashes horribly with the bright red
but alas, it's the category that's worked best for me so far, so I'm sticking with it.
Other WT categories I tried:
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Thriller: I liked this one. It had a nice grey banner which DIDN'T clash with my thumbnail AND it had its own Top 30 list, unlike Crime/Mystery. However, when I switched to that one, it felt like I was getting no new readers at all, so... bye.
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Fantasy: I mean, it -can- work as a secondary genre if you happen to post at the right time. Problem is finding what the right time is. Because otherwise your comic is gonna disappear after 0.2 seconds
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Crime/Mystery: took me some time before I eventually decided to try this one. First, I didn't like the "crime" part, because it felt more appropriate for a detective story à la Sherlock Holmes or a heist story, and my comic was none of those. Also, there's no indipendent Top 30 list for Crime/Mystery. But it IS a pretty small category, and your comic stays in the recently updated list for a very long time. So I went with that as my secondary genre.