As the official self appointed Em-Drive denier here, my recommendation is to provide more information in the comments. Sometimes it's difficult. Most commenters here are not experts in the particular field, but are curious. Sometimes it's better to center in the worst part of the comment if it has too many errors.

Sadly, I don't know enough cosmology and general relativity, so I use this approach to this problem https://xkcd.com/793/

I think that general relativity give minimal corrections in the scale of a galaxy, only some local corrections that are important near black holes and some spots. So it's enough [citation needed] to use Newtonian physics, perhaps with some correction near the galaxy center and perhaps a small effective field with the average distribution of matter.

To track an individual star during many galaxy revolutions it may be necessary to be more careful, specially if the stars pass near another star. But to have a good estimation of the rotation speed it is possible to almost ignore general relativity.

Question time: Do the current simulations in the area use special/general relativity corrections? Do you have a link to a recent paper? A link to a simulation program? [bonus point if the link points to the option page to enable/disable relativity or select the approximation method] Is there a Wikipedia page about this? [Most of the times the Wikipedia page is easier to understand than the paper.]