[C]apabilities to feed sheets .... I was planning to make 6x8 proof prints but the 0.79" minimum rear border is an issue.
Not knowing what the other minimum margins are, but e.g. if they're 0.79" all around, it seems like you could readily print 6x8" on letter or A4 sheets, which are widely available in almost any photo paper. The former would leave borders of 1.25" each side and 1.5" top and bottom, the latter 1.13" each side and 1.85" top and bottom. That doesn't strike me as massively wasteful. Obviously if you can gang-print several images at the same time, then various other sheet and roll options open.
(Of course, e.g. printing on letter-size you might prefer to use a bit more ink and make the 4:3 proofs 6.75x9", or on A4 6.6x8.8".)