are there any all-CRNA anesthesia groups?

AANA practice surveys show that about one third of CRNAs do not work under MDA stupervision. Rural America is served mostly by CRNA-only groups or by solo CRNAs.

Wouldn't go that far.

While the miracle center facilities will most often have departments headed by MDAs, exceptions do occur, such as ... OB epidural service may be contracted out to a CRNA group ... or, plastic surgeons (and other specialists, like ophthalmologists) who own their own ASCs often deal exclusively with CRNAs. And there is office work as well, such as dental sedations. All very dependent on the local situation.

Not categorically clear-cut urban/rural, no.