I have 50 plants - throwing them away is not an option.

Why would you do that?

I really can't understand how some people advise/think to throw away sick plants when these are excelent opportunities to learn.
Obviously there's some cases where this is the only option but only when all the other possibilities have been tested.

Again, use imidacloprid and you'll see improvements in a short time.

The roots (there they multiply) are infested with scale, so you have to change the soil probably also being infested with a fungus or virus. Next year you will not see flowers from the plants. The insecticide also costs and imidacloprid is a neurotoxic (destroying bees is banned in many places around the world). It's an option, I have it, and I have learned it.
How could 50 plants be infested and you do not notice?
Probably the least infested you can treat, but with very infested you will have to learn the lesson. I personally grow orchids for me to enjoy when they bloom.
I think with $ 10 you can buy a healthy plant.

With 50 badly infested plants your only hope of success is to use a pesticide.

I'm very light handed with pesticides but in this case, I'd spray and spray diligently for a month or more. Granular would be great for maintenance.