The project for that stream was built in 4.19, works in 4.20, and I recently tested it in 4.21.
So Start Time should work now.
For your case, I would build a little stem manager and use the Wave Param node to pass stems in and I would not do any internal looping (kind of like my Ambient Music demo later on in the stream). Then I would be maybe playing a handfull of stems at any given time and I would just reuse the same two SoundCues to alternate between them.