@allysmiles3 I feel your pain. If it makes you feel any better, you're not alone. Here is my story.
https://www.reddit.com/r/naturalbodybuilding/s/iqQV6iQZ43
Almost identical. I can gain and lose weight on command, and I have done 3 cut and bulk cycles with different programming. I program for progressive overload and push to failure, but can't increase lifts or muscle mass over the long run. The only thing that goes up or down is my body body fat %. Also 6 foot, weight from 145-175lb. I have done dexa scans, body measurements, and hormone testing Still no good news for me, though.
Even if your programming was mediocre (which I don't think it is), you should have increased muscle
Hashimoto's may have more effects on your progress than you think. I haven't researched it much, but it's a thyroid disease that affects hormones.
I would talk to a doctor, endocrinologist, and get your hormone levels tested if you haven't already. I would test testosterone, but others too. Cortisol, estrogen, sbhg, igf-1, thyroid, dhea. Also, you may be taking medication that would have an effect on muscle mass. If you haven't, do dexa scans and body measurements to ensure you are not actually gaining muslce. If your measments go up but your waist stays the same, you're gaining muscle. Also, have someone objectively look at your pics to ensure you haven't gained any muslce. It's best to compare apple to apples, so at the end of a cut vs. an end of a cut.
As you can tell, I've been down this road, and I haven't given up yet. Lifting has improved my life in many other ways. But it's tough to stick with anything for years without getting any results, especially when everyone around you is trying half as hard and getting better results. Feel free to reach out to me, I'd be happy to get more into detail and try to come up with other solutions together.