How does career progression look for a 91D - Tactical Power Generation Specialist? How hard is it to make it to SFC?

You are much more likely to achieve SFC as a 27D then a 91D.

91D is a terrible MOS for rank progression. You will remain an E5 while watching your peers in the motor pool progress to SSG, SFC and Warrant, all while waiting years for your points to drop.
If you don’t want to be a 27D and you’re looking into the maintenance field I would highly recommend 91B which often does the work of a 91D. 91A and 91S also have good promotions but limited duty stations. 91B - 91L convert your 91X at SFC, meaning all those other MOSs are competing against 91B for 91X spots, and the 91Bs are overwhelmingly selected.

SFC (Join to see) Maybe you could share your career counselor wisdom for this soldier.