Sacrificing only Fjorm for Atk/Def Bond?

Fjorm can be put on quests that requires her to kill 'x' units or something.

Unless you want to miss out on those, then don't do it.

I would strongly advise against it. You need Fjorm for missions and for arena as a bonus unit. You'd probably be better off waiting for one on a legendary banner than foddering her off.

One of the downsides of sacrificing the F2P Fjorm is that you lose perhaps one the units that's consistently used in F2P guides for specific maps.

If you don't care about that, then you could always consider Fury 3 and try it out before committing to Atk/Def 3 bond. Having it on Myrrh means you'll need someone consistent enough to always be adjacent of her. And while Fury 3 does have recoil damage, it's still a useable budget skill.