Which product (concentration) are you using? Also please describe you application process to see if we might have tips to offer for increased efficacy.
I've never had to wear protection, yet.
How many birds do ya have total besides the two old gals?
I would consider switching to a "grower" or "flock raiser" with 18-20% protein and just keeping oyster shell on the side for active layers since she doesn't need the extra calcium of "layer" and those both appear to be dual purpose breeds which will do best with not less than 18% protein. Aside from extra protein the flock raiser also contains more amino acids which is key. Alfalfa is also high in calcium I believe. I might switch to using that as a treat instead of mixed into the feed.
Since she's active and just a tad slow, I would likely keep her with the flock as it may be less stressful than separation.
Eating slowing down with age is actually normal. Metabolism slows as well as activity lessens... so they need less calories to meet their energy level.
If you have Epsom salt to add when bathing her... it can help to be relaxing... the magnesium absorbs through the skin and into the muscles helping reduce soreness in athletes. Drinking it could cause diarrhea though, so try not to let her.
It is important to remember that chickens being prey animals will hide there discomfort just as long as possible... also to avoid their own flock from possibly pushing them out or cannibalizing them in an attempt to do so.

So paying attention to the things mentioned by Wyorp are key... eating, lethargy, flock dynamics...
A life quality assessment tool...
https://vet.osu.edu/vmc/sites/defau...ionAnimals/HonoringtheBond/HowDoIKnowWhen.pdf