Until last year, I made a lot of Paranormal Romance covers for people in my writers groups. I kind of burned out on those, so I'm mostly staying away from romance covers right now, including for Vella. There's no point in doing the best-selling thing if you're starting to hate it... especially when other genres are still fun!
I do read a lot of serial fiction, yes. On RoyalRoad, on various manga scanlation sites, fanfiction sites (including AO3 and Wattpad), etc. Romance isn't the biggest genre on all, not even close in some cases. RoyalRoad, for example, is more fantasy-centric. Especially Litrpg and similar progression-type stories are extremely popular there, no matter how you measure popularity (total reads, total followers, total monthly earnings on Patreon, etc). Those have the advantage that they can go on for millions of words and hundreds or thousands of chapters. A lot of my pre-mades were made with progression stories in mind, but could fit other genres too, since Vella doesn't have fixed genre conventions yet.
We won't know which demographics will actually end up reading on Vella for a while yet, but I'm not sure romance will necessarily get the largest share. There are large swathes of older teens and young adults who are vocally tired of romance and even stories involving romance as a side plot, so I could see a large market share going to romance-free litrpg stories. Especially since avid romance readers already have a KU subscription or own an e-reader - why would they switch to reading on a phone, or paying extra for Vella? Vella doesn't seem designed for people who read a romance book in a day, since it's specifically bite-sized chunks. This seems more aimed at people with short attention spans or not much time to read. This is just my speculation though!
You're right that subtlety isn't usually popular for serials, though it can work for sequels once you have a large following. Symbolism on the other hand can often work, especially if it's somehow cool or stylish and not too complicated. Image C10, for example, could work for a progression story where the MC is a strategist and rises to the top through his smarts. C4, on the other hand, would fit a story where a group of people get isekai-ed together (they don't even need to be traveling by bus when it happens!).
Edit: I forgot to say, I'm not sure some of the typical action poses will look good on Vella covers. I saw some vella covers for sale on etsy that were very mis-centered. They looked good square... but when presented in a circle, the way Vella images will be shown, the top of the hair was missing and the thing that was dead center? Boobs. Now, boobs can work for some genres, but if they aren't what you intended...
On a regular book cover, you end up looking near the top first, which hits the face in many action poses. On a round image, your view goes more to the middle, which consequently hits the boobs. You can of course make the person smaller to avoid the bullseye cleavage, but then you run into the "is it too small to be clearly visible" issue.
This isn't an issue with manchest covers, but Urban Fantasy action girls? I think we'll see quite a few mis-aligned covers until Vella's genre conventions settle down and cover designers get used to the new format.
Second Edit: if you want an example of how much "designed to be viewed on a phone screen" can change a whole industry, check out some webtoons (first invented in that format in Korea, then quickly spread) and compare them to western webcomics. Regular comics, including most webcomics, have the familiar page layout with multiple panels. Webtoons on the other hand? Are designed to be read on a phone screen while scrolling from top to bottom.
https://www.webtoons.com/en/horror/tales-of-the-unusual/list?title_no=68&page=31 This is Tales of the Unusual, a super popular horror webnovel. It's a series of multi-chapter horror short stories. Even if you don't like horror, take a look to see how different it is made compared to regular comics.