I'd be surprised if there are any successful ones out there. Any successful promotion service runs on affiliate earnings - and KU only pays you for a new signup for a membership. In the customer journey you do not start out with the KU promo site - you subscribe once you are in KU. In DE amazon pays 3 Euro for a new KU signup (likely only once in the lifetime), while each sold book - even discounted - earns a few %.
If you want to promote more read, you are probably better of bartering with other authors for newsletter swaps and cross promotion.
KU readers are likely voracious readers and there is always a place to find more of what you like. If you can identify what readers like about you, you can target similar authors and try cross author readthrough.
I 've never seen promotion for other authors outside of box sets in books, but that is usually "meet 10 other fantasy authors". What readers are looking for is more of the same - not just the genre. It is work, but if you can set up an intelligent funnel for the KU reader there is a good chance of success.
Because once you read a lot, you will have read most of the authors suggested to you, the also boughts are useless and searches bring in already read or unwanted content. Which is when they start to look elsewhere.
The key for you is to make it evergreen as possible. Ads stop working once you stop paying for them. A newsletter swap likely is only for a short time. So what is a way for a reader to discover you through other similar authors? Chances are the reader will read them all much faster than you can write anything. hth.
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