JDW - this is hero's work. Very informative of the gritty details of capacitor failure parameters. I regularly teach physics class with too many theoretical concepts in not enough time and sadly, we never reach a point where we have time to discuss real world issues with circuits.
I recapped my own SE/30 in the summer of 2019 and I was also led to believe to mostly take above-spec voltage rating caps, especially for the problematic one (C1?) in the Mac Plus analog board. I did take 19 of these to get extras for my Classic 1, but mostly to recap my logic SE/30 board:
I was rusty at soldering, new at picking exact BOM parts, etc. I'm wondering if my choice is mostly ok. I think so. If I'm understanding correctly what you wrote in your OP, my 20 V can tolerate 16 V (80%) correctly, but that might push things at the edge if I'm unlucky.