Correct voltage of hot wires with common neutral
Having had to rewire a factory after someone stole all the breakers and all the wires up to the edge of a panel, I see it in the converse manner as ThreePhaseEel.
I don't say red and blue need to be on different neutrals.  I say red and blue breakers need to be on opposite phases.  I.E. the breakers are mispositioned.  I wouldn't do anything to the garage wiring, other than understand it.
I suspect it was wired correctly originally (albeit lacking the handle-ties now required) and then some years ago, a previous maintainer rearranged some breakers and did not realize these were MWBCs. .
So I would reposition the red/blue breakers back to where they belong: on opposite phases... and declare victory, myself.
In other words, I'm saying &quot;fix it at the breaker panel by correctly phasing red and blue&quot;... and don't &quot;try to rearrange the garage wiring to change how the MWBCs are grouped&quot;.
For right now, swap the 3 breakers around so red/blue are opposite phased.  It's not a handle-tie, but it's what can be done right now.  (I assume you have a panel full of tandems/double-stuffs, and those get VERY confusing).

However, separate from that, it's a Code requirement that the neutrals be identified in association with their partner hot(s).  Things like that are why my electric toolbox has 10 colors of tape.  I would simply tape the white wires with the colors of the hot(s) they support; there's nothing wrong with that *when it's individual wires in conduit; as remarking individual white or gray wires does not convert them into a hot.
So wrap them or tape them as pairs/triplets, at both ends.  The ground is shared and doesn't need to be associated with one or the other.