This may seem extreme but I’ll just put it out there in case it’s an option you would consider. My cousin’s child had a very rough junior year last year. Went from straight A’s and 99th percentile test scores to poor grades and struggling. There were some health issues but I don’t really know all the details and I don’t think that was the whole cause. After much hand-wringing they switched to a private school and repeated junior year this year. I’m not sure what else they did to address the issues they were having. But good news the kiddo is back on track this year making good grades and in a much better place for college applications for next fall. Again I realize repeating a year may seem more extreme or may not be an option for you but just wanted to put it out there for thought.
Good luck to you!