Based on some of the results I'd seen on Kboards especially on this thread ->
https://www.kboards.com/index.php/topic,184950.0.html, I booked a free book promo with Bknights on Fiverr, for my permafree novella . He/She/They

gave me a confirmation for June 2nd, and I have to say, I was impressed with the results.
Here's an image showing the spike in free book sales during the promo.

The highlighted number is the number of free books purchased on the day of the promo. My ranking in the free store rose from about 2000 to 114. In any other genre but romance, that would have been enough to put my book at the top in it's category, but romance is slightly (a lot) more competitive than other categories, so I never managed higher than about 83 free in romance.
Subsequent sales have been steady. I had a new release on May 14th, so I can't say for sure if the level of paid sales at the moment is due to the free promo or to the new release.

The first spike above was on the day of the new release. The days after the promo don't show such a sharp spike, but I'm sure the free promo has had a hand in ensuring that the surge in sales due to the new release hasn't tapered off.
I'd totally recommend Bknights for a free promo. Response time is fast, the service is awesome, and you can't beat the price! I actually got the Kboards special, which is a discount you get when you tell them you're from Kboards, and I got better results from them than I did from another free book promotion site which charged me $15 for a spot on June 3rd, and never quite managed to match BKnights.
I'll just add that, if you really want to get into the top 100 free, I'll recommend combining Bknights with another promo on the same day.