+1 to overbuilding and saving starters, here's what I usually do.
Make a starter with an extra ~50B cells. Once the starter is done, I swirl it all together so it's roughly homogenous, then pitch all but the percentage of the volume 50B cells takes up. Alternatively if I'm building the starter in advance, I'll cold crash, decant then save the percentage of volume of the dense slurry.
I'll usually save that bit of the starter in an 8 oz mason jar. Since I've switched to mostly kveik yeasts, I've also taken to drying some of it in my toaster oven (which goes down to dehydrating temperatures). Dry on lowest setting (100F for me), on parchment paper. Crumble up, pour into ziploc bag and freeze for long term storage. If something goes wrong with my liquid culture, or I just feel I've been using it for a long time, I'll refresh it from the dried culture.
With that said, I don't recommend this if you have cheap dry yeast forms, now that Lallemand has a dry form of Voss, I'll probably try out for ease of use.