I admire your "restraint". You don't plan on buying 20-30 years worth!
Before this question, I was planning to use the "buy as needed" approach for a 2020s 10-year restraint. Now, I decided to use the "desert island" approach. 24 pucks of Williams and 200 Astra SP's are all I need to buy. I might just make it to 20 years on the soap. The blades should last closer to 30. Me? Who knows?