I don't know of any authors who write similar books to mine. My Leon Chameleon PI books feature anthropomorphic animals and I have likened them to Beatrix Potter or Kenneth Grahame, but I doubt if many readers search for those names. Categories will reach the most readers.
My books are chapter books, but BookBub only have 'children' and 'middle grade'. Their middle grade requires a minimum page count of 100 pages
and my books (trad published) are about 60 pages.
You might find ads that still work well with smaller size books (but it sucks that their editors are not accepting them for promos though
).
Ads are expensive, not only due to the ad cost themselves, but the testing needed. I wouldn't put too much thought over the authors on your BookBub ad tests. I'd throw lots of darts out and then take a look at the data. It doesn't matter if an author writes novels or chapter books. If the author you choose in your ad leads to readers, or most importantly, buyers, to click on your ad, then you'll start seeing success.
One of the main reasons why many of my BB ads are so successful is because I know the audiences that will
look at my books. Realize that you want to grab readers to
look at the ad, NOT cater to readers that you think match your books. An author that you think fits your writing might not work at all for bringing your ads clicks. One author I stumbled on yields me a click rate of 5%, sometimes 9%, but I stumbled after trying out multiple author, I didn't research all that much. And it was after running upwards of a hundred of ads with other authors.
So set ads at $5 and watch your click rate and impressions. You start BB ads with a high bid to get bites, and then work your way down. My ads average a click rate of about 3-4% which enables me a fairly good payback for full priced books. When I started BB ads they were at 0.5. If I don't get sales, I stop running the ad. Hope my advice helps, Jan. Good luck!