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...