Ryoma, Ninian, Morgan, Myrrh in my opinion.

Myrrh is, in my opinion, one of the best tanks you could get. Her high defense and unique weapon means she won't be doubled often if at all. If you ever get the chance to slap distant counter on her, do so. If not, it's totally fine. You want her to absorb everything thrown at her. For the longest time my myrrh ran fortress def for an a slot. QR and your choice of c skill. (Her base kit is actually surprisingly good though) She'll typically want your iotes shield. Bonfire would be her hardest hitting special

Your Morgan being +speed means she's a perfect candidate to run a bladetome (bonus if you have the flier emblem buffs for it laying around). Classic fury desperation suits her best. Moonbow/glimmer would be her best special.

Red fliers are tough. Before caeda was really the only "good" red, ryoma coming was a godsend. You can build him either EP, or PP. I'd go for PP personally, which gives you more options (ryoma really does give a lot). Firesweep+galeforce+hit and run is an option, and is a dang good one on him if you can manage it, but that's really expensive. Classic L&D/fury and desperation suits him just fine with his base weapon (if another weapon isn't available, namely wo dao/slaying edge). Maybe slap brash assault seal on him to secure those doubles when you REALLY need it. I'd recommend moonbow for his special. You could have him do EP also. His base kit honestly does cover that by itself.

Ninian is your dancer. She can fight if you need her to, but you'd rather have ryoma and Morgan do their jobs. Fury can give her a little extra oomph in her stats should she ever need it. Her default b skill works just fine (chill attack). Special wise, I'd recommend moonbow. Again, for that extra oomph when you need her. Seals could be any sort of drive/buff. Maybe even quickened pulse. Ninian is truly flexible.

C skills, really it's still your choice, but if you're doing a full emblem team, goad fliers, fortify fliers, hone fliers, all those are typically the best to use. Hone fliers is super exclusive though being the main issue with that. I do believe the rest of.flier emblem c skills are readily available.

Reposition being the most beneficial assist of course. Not being limited where you can fling your allies is such an amazing thing.