I have 5 stars I absolutely never ever use. Advice?

I'm not gonna blame you, as Im the one with karla, ayra and mia with perfect IVS at lv1, along with a lot of "top tier" units at lvl40 with base kit (if even the sp reached at lv40 was enough), I only use chars that I like.

Now back to your question, azura, marth, myrrh and L!Lyn should have good synergy: all colors, all roles (dancer, buffer, carry, tank), can use tactics... but you have multiple good combinations with all those char

A little late, but I'll post anyway

I will not take into accound dew cost, but all the surgestions with refinable weapons assume the use of em.

You cold make a rather solid flying team out of this:
Elincia as main (melee) offence
Sumia and Myrrh as good defencive units.
Add in a mage flier like Aversa and you've got a team.

Chrom and Legendary Lyn can work well together. They both want allies close. Chrom of course with bonds, Lyn can be a great anti range with laws of sacae and distand def.
Azura can be added for positioning. Maybe an offence mage to finish it off.

And a few ideas for seperate characters:

Marth is great in many teams as support. The drives on his weapon combined with some in his C and Seal buff up his team massively. As a bonus he can heal people with reciprocal aid and his falchon's renewal

Tiki is a great dragon check on dragon teams. And has great def if used well.

Olwen, while not a great unit, is an option for a owl or serpent tome. Her speed saves her from doubles, so add in some distant defences and quick ripotose and she'll bait well.
She'll still have rather low attack though