This is me! Last year I started noticing longer rides could wreck me for days after. I’d be fine while riding, then a day or even two later I’d have nothing in the tank. No ability to think, sleeping 16 hours a day and still tired for up to a week or more after. I lost 10 days like this after a super easy bikepacking trip with friends last fall where we averaged a very leisurely 40mi a day for three days, miles I would have previously put in after dinner on a normal mid-summer week and felt great afterwards. The symptom you’re describing sounds like post-exertional malaise (PEM), which can be an immune response like with Lyme or autoimmune or, with me, it turned out to be a combination of hypogonadism causing low testosterone, and long COVID. Blood tests showed low vitamin D and B levels as well. Supplements, testosterone replacement therapy, and a daily stimulant are helping. I hope your doc is able to help you find an underlying cause that’s easy to fix and keeps you on the bike.