Dimension of orbit versus invariant functions
Yes for $G=\mathbb Z$, see Theorem 4.1 of this paper by Amerik-Campana.
Following citation links from the paper Jorge directed me to, the case of general $G$ is done in
Bell, Jason; Ghioca, Dragos; Reichstein, Zinovy, On a dynamical version of a theorem of Rosenlicht, Ann. Sc. Norm. Super. Pisa, Cl. Sci. (5) 17, No. 1, 187-204 (2017). ZBL1401.14071. (arXiv link)
Disclaimer -- I have only read the MR review.