Completely agree with Wendy.
I know you don't think much of set nap times for Caleb, so if you don't want to go there, that's ok. But you've tried so many different A times and are getting so frustrated. It's not worth it, in my book. I think if he's not teething or ill, set that nap time and maybe bedtime too and see how he regulates. At 17.5 mo, this is what I did with my LO, and it was pretty easy to see what times were going to work best for her after a couple weeks. I think this is the only way you'll get to push bedtime back eventually.