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 "fix it at the breaker panel by correctly phasing red and blue"... and don't "try to rearrange the garage wiring to change how the MWBCs are grouped".
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.