We have one property, a big cabin (sleeps 16) with a $375 cleaning fee. We give our cleaners a $50 bonus if the guest leaves a 5 star review (overall). That's a 13% bonus and it's a nice round number that feels like a substantial bonus.
I don't have any historical data since we've had the same cleaners and we've only been live since December 2021. But I can say that they do an incredible job and have a sense of ownership in the house.
Every stay has been 5 star except for one 4 star (when we started). Every Airbnb review is 5 stars. For some reason we get a small minority of guests that don't review on VRBO (I suppose because those guests are older and less tech savvy).
Additionally we plan on giving them a big Christmas bonus this year because we appreciate the good work they do for us. There's no math or incentives involved. It's just gratitude.
As for your strategy, the first thing I thought was that I wouldn't be motivated by a $5 bonus. But I don't know your market and pricing.
I like 5 star review metric bonuses (after all, a clean house is the foundation of a 5 star stay) but it has to motivate your cleaners. Increase the bonus or perhaps lump together 2-3 5 star stays into a larger bonus. I don't know. I'm just saying if I was a cleaner and I got a $5 bonus it would not impress me (and in fact may do the opposite).