All excellent points. I'll be elaborating on this more in the followup post, but I find that it makes a surprisingly big difference for the overall effect of the poem whether the post feels comfortable varying the refrain lines. When the poet is not, yes, definitely, philosophical unpacking or an obsessive circling flow is very apt; when the poet is comfortable making minute changes, then I tend to see poems that look to surprise the reader in some way or overturn expectations, that build their force on repeating, repeating, repeating and then finally changing the repetition such that the poem breaks out of that obsessive cycle into new territory.

I would love to hear more about your thoughts on sonnets - it's not a format that I've used myself.