I just searched the type of mite and it actually looks like poultry lice rather than mites.
The article below is a good one to id with.dead red mites were all over her! I would say hundreds of them, maybe more.
Either way - Lice/Mites (?!) if she's infested with 100's of them, you need to treat her and likely your flock/housing as well.
https://the-chicken-chick.com/poultry-lice-and-mites-identification/
If she's infested with Red Mites, then that could make her weak, but she most likely has something else going on - she's 20 weeks old so *should* have been dust bathing/active which *should* have helped reduce the parasite load - but she's been ill for what 3-4 weeks?
I would get the B-Complex or some poultry vitamins like Poultry Cell to give to her. You can pick up some poultry cell when you go to the feed store to get the Permethrin spray/dust.
Will she not eat feed on her own? I would make her a wet mash of feed - if you have chick starter that would be good, if not then use layer feed.
Since she's infested, then I would treat all the other birds and their housing too.