Your experiences with GLP1ras are in keeping with what I’ve heard from many people.
They inhibit hepatic gluconeogenesis… which is an important source of muscle fuel during higher intensity exercise. Which is my guess why people notice a drop in ability to do high intensity efforts right away (that immediate drop in FTP you noticed).
Muscle loss is a know side effect of them as well - depending on the study, up to 50% of the weight loss seen is due to loss of muscle (not just fat). We are actually starting to see the rare person develop marked sarcopenia as a side effect of them.
I remember there was an old thread here where people were asking whether these would be abused by professional cyclists. Your experience is a great example of why that is unlikely to be an issue.
They are helpful when losing weight is the single most important thing on your plate to improve fitness and health. Otherwise, not so much.
Congratulations on your successful weight loss journey!