I would vote for Miki - he is a game changer for very fast wars and he benefits much more than the other two from the lb (generally attack heroes benefit more from lb than support heroes imo)
In my honest opinion, Miki will work really well for you without the LB, so save the aethers for another.
Additionally I am not a believer that strong healers need LB if they already have strong stats.
So that leaves you Frosth - who never really poses a threat
OR
Wait for another worthy contender - my view is don’t rush to allocate your aethers on this occasion.
But - whatever you choose to do, I wish you luck
Good heroes you have there. Out of all the heroes you listed, I think I’d limit break Richard (especially if you have his costume).
But otherwise, I’d probably wait as some have suggested. I never see a rush to allocate aethers, they can be really hard to come by (especially 5* aethers).