I had similar problem before and the main issue was that I was supposed to start Buserelin on D21 (after ovulation), but that cycle for some reason ovulation didn’t happen as usual (I tracked it with tests) - which means once I started Buserelin it sort of suppressed the ovulation hormone and I actually never got a positive ovulation test that cycle. In the end my period did come but super late… and after that baseline was fine with thin lining. I was quite annoyed tho because it meant so much more injection compared to other cycles.
I'm actually in almost the same position! I had two scans and at both my lining was too thick and I also had follicles that were measuring on the bigger side. After my second scan, they got me to take half of my trigger dose to make the large follicle ovulate and I've kept up with buserelin in the meantime. They told me to get in touch when I bleed again and I'll have another scan a week after that. I can't say yet whether it will work, but just wanted to share so you know there are options! I've been on down regulation injections for 43 days now, so I feel your pain! Best of luck x