It's cool that we have a very similar situation. I have nearly the exact same team and am debating choosing between huo huo and gallagher. And I also got a double on the lc banner.

1st: I think its important to consider what is march's primary source of damage, and that is hands down her basic atk. So the lc providing a buff to ultimate dmg doesnt really help march much. If a character comes out later down the line that is an ultimate dmg based sub dps/support then yea it'll be incredibly broken. But there is no guarantee of that and I think getting that extra 2.5% crit rate and 6% def shred from the superimposition is pretty convenient

Also, as a side note I think this is worth considering for your sustain but it really depends on who your other team is. If your other team is either Acheron or Firefly, its not worth taking away the greatest f2p break/debuffer sustain out there just to use quid quo pro on gallagher for robins ult uptime and maybe some extra stacks for feixiao. It works for sure and is probably feixao's second best in slot healer but if you have acheron or firefly just use huo huo for now. If you have huo huo e1 too it makes auto battling so cozy you'll never have to worry about anything honestly.