Regarding the loading times.

Still faster then waiting for musicians to show up at the studio. I haven't timed it, but I think under a minute to render the project ready to play. Computer processing power and speed certainly does make a huge difference in this area.

I haven't used the "regen" to redo a section of the RT.

I simply render the track using the same RT multiple times. 3, to 5 times is generally enough. I load them into my DAW stacked together and use envelopes for volume control.

I did this on my song The Best Christmas with the piano fills (3 tracks) and the guitar solo (5 tracks). It took some time to set up the envelopes but the end result was worth the time spent.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.