Firstly… Ascend hero’s for attacking utility first, not just for your defence team.
Beyond that… Vela works best as a flank on Telluria. She’s a good all round hero too - definitely go ahead and ascend.
JF works well at flank and BK has been doing the business for me… But if I were you I’d pick one or the other with Vela on the other side.
Unless you have a deep red bench to take advantage of JF’s crit boost, I’d consider waiting to ascend him… He’s good, but you may find other hero’s more useful to do first, especially as your existing red 5* are both supporting, do you really want another semi-supporting?
Rainbow generally works best anyway and fast sniper flanks are the ticket - they work great on defence and they’re also hero’s you want to be ascending for attacking raids/events/maps anyway.
Seshat and Joon will do the job just fine
Seshat - BK - Telluria - Vela - Joon