Playing devils avocado here, why not just chuck in two packs of dried yeast?
Imho the 11gms supplied as standard is a bit light on (it's interesting that Coopers are now selling dried yeast, but in 15gm packets, don't get excited it's all generic stuff)
With the recent advances in dried yeast production, and the ever growing variety I question the value of liquid cultures for every day use, sure they have their place but factor in the cost and time associated with them, and the argument is far less compelling now than it once was.
Find a specialist supplier with high turnover, I've had great results with
Cheap Yeast Store no affiliation blah, blah, blah. He's often "sold out" (that's a good thing) and has a decent selection.
Having said that, I do maintain a frozen yeast bank, I pretty much use Kadmiums method but draw off the yeast from the 500ml into 50ml centrifuge tubes with a syringe, and store in a glycol mix.
This is only done with the original smack pack/vial, as MHB points out there are bugs in yeast, the yeast manufacturers include a bacteria count in their data sheets, it may be a tiny amount but it's there, and they do multiply, wont argue that you can't get several generations out of a pack but the risk increases with each one, and genetic drift will ensure that the beer you made with gen1 will be different to the beer you make with gen10, it might still be great beer but the yeast will have mutated to a greater or lesser degree depending on the stresses endured by the multiple fermentations.
just my 2 bobs
fwiw if I'm sterilising an erlenmeyer with wort in it, glycol centrifuge tubes, syringes etc. I use a pressure cooker, just fill with water to about 2/3 the height of the wort and let it whistle for 10 min's, apparently there's bugs that can survive 100c, and 125c at pressure kills them.