MarkB
Legend
That part would be a prerequisite for me even considering it. Otherwise it's too easy for the familiar to just run and hide.EDIT: One balance possibility is if either you or your familiar are forced to make a concentration check, the other must also make one. So, you are potentially losing both spells for one damage source.
My suggestion would be for you to come up with a set of additional familiar utility features that can be gained by upcasting Find Familiar. This one seems worthy of, say, a 4th-level casting. This would also have the effect of limiting the ability to dedicated spellcasters, since the spell can't be cast at a higher level as a ritual, nor using the daily casting from Magic Initiate.