For what its worth, I followed Randy's recipe to the letter and tried it over quite a few hives in Summer 2020. The results were disastrous, with extreme varroa damage. Randy thought that humidity in the UK might have caused the ineffectiveness. I'm still dealing with the hangover of rusted wires in frames (vertically wired frames...horizontal ones would be fine) and a rusty reminder on queen excluders of where the swedish towels were! Its a shame it didn't work...could have been a silver bullet. Whoever finds a reliable method of killing varroa effectively with no contamination of supers in place will be a popular chap/chapess.
Good luck with your experiments. I tried little or no treatment on new colonies between 2018 and 2020, relying on visual clues (ie couldn't see any varroa on bees) which is not a reliable way of assessing load (Mite wash is), so by the time I tried Randy's method, the hives were probably pretty well infested, although not showing clues that were obvious to me at the time. With hindsight, I would say that brood patterns had deteriorated, but I had got used to seeing sub-standard patterns so believed all to be normal. New colonies generally cope for a couple of years, after which deterioration of brood pattern, yield, colony size etc gradually becomes worse, with the optimistic beekeeper maybe putting these changes down to other causes, or choosing not to see what he/she doesn't want to see. It is the third season where the chickens really come home to roost in any that have survived. Lesson learned here. If you are going to experiment, try various methods on a few hives and be consistent about mite washes, otherwise you'll have no really idea on how effective any treatment is....until varroa population explodes.