> let's switch to an interstellar time standard in a rest frame relative to the CMBR as far outside of gravity wells as possible
This won't work: since galaxies aren't static, and because the universe expands, the point with the highest gravitational potential ("as far outside of gravity wells as possible") will both move w.r.t. the CMBR and have a gravitional potential that changes over time.
Maybe I'm making a bad assumption that intergalactic space is so flat that pretty much any region of it will do as a reference. Since we can't reach those regions yet we'd still be approximating it with our slower clocks, but it seems like it's at least feasible.
Galactic groups are gravitationally bound enough that they’ll stay together through the expansion of the universe. Would want to find an area that is particularly void of galactic influence maybe?
This won't work: since galaxies aren't static, and because the universe expands, the point with the highest gravitational potential ("as far outside of gravity wells as possible") will both move w.r.t. the CMBR and have a gravitional potential that changes over time.