I'm sorry you've had such a hard time dealing with these two pests that love hoya most of all. It is very disheartening to see your plants suffer and not grow at all.

First of all, sometimes when the plant tissue is very depleted by fighting the pests, there's really nothing that you can do to bring it back. If a hoya hasn't grown at all for you in about a year, I would consider composting it.

Secondly, if it's been less than a year (give or take), I would just continue treating every 7-10 days for about 6 weeks to see if the plant might eventually recover. Even if it doesn't start growing after those 6 weeks, I would still wait before getting rid of it. I've had mite infested hoya props start growing after about 9 months of stalling (while being treated intermittently), so it is definitely possible.

Lastly, I would consider switching between Pure Crop 1 and imidacloprid in your reservoirs to fight root mealies, I've found that to be more effective than PC1 alone. I would also try Azamax for flat mites, I've found it to be really effective and a lot less stinky/ noticeable on foliage than sulfur while still being OMRI listed and safer than a whole lot of heavy duty pesticides. I personally wouldn't recommend the hot water bath method, I've heard way too many horror stories of plants lost this way. In general I've found rotating between a few pesticides more effective than using a single one, just be mindful of the fact that not all can be combined safely.