I have heard and seen "only ever" used as in: "I only ever clean my car when the sun is high." I live in Massachusetts and never heard this usage until recently. I understand the meaning, but wonder if this is a regional usage.
I, too, have lived in the South all my life--I'm 73--and have never heard "only ever" used except in books. It sounds awkward to me, since just the word 'only' means the same thing.
"Ever" is an intensifier, with its proximity to "only" adding to its effect. Grammatically, these mean the same thing.
"I only ever clean my car when the sun is high" "I only clean my car when the sun is high, ever."
It functions similarly to "at all"