Great thread. Thanks for the experiments and reading, definitely going to be using this.
Any idea if adding trehalose would help the viability, and if it does do you think you'll be able to shorten the fridge step due to this.
This thread might be relevant for you:
https://www.homebrewtalk.com/f163/do-you-know-how-make-yeast-starter-then-why-not-farm-yeast-freeze-269488/
https://www.homebrewtalk.com/freezing-yeast.html
If I can remember correctly, the TLDR is:
7.5% glycerin/yeast slurry performed best.
Cool slow if...