I've never used an outside beta reader service. That's my disclaimer. I've been fortunate to know people who've been willing to read my manuscript. So I'm unsure of your new service (though I'll mark it for consideration myself).
That being said,
if I was to seek beta reads out, I'd consider Hidden Gems and vibe reviews.
Hidden Gems:
https://www.hiddengemsbooks.com (note Hidden Gems specializes in romance)
Vibe reviews:
https://www.vibebookreviews.com.
Why these services? Because both services have been very helpful as resources for honest ARC reviews. I suspect they use a similar pool of readers for a beta read.
.. I was hoping to find a way to safely distribute beta without being tied to Kindle.
Alternative would be to send as epub, mobi, or pdf file through Bookfunnel. Bookfunnel charges $100 per year but they watermark. I use it to send arcs. I also use it for my newsletter. But, honestly, bookmarked or not, there's always a risk of piracy so tread carefully--particularly with unpublished material.