>any "on" time is costing me usage in the long-long-long run and I'd rather have my monitor last 5+ years than ... 2 or 3.

Going from dark gray to pure black isn't going to halve your monitor expectancy, if it makes a difference at all. Due to how human perception works something that's merely dark gray is actually orders of magnitude brighter than pure white, or even 50% gray. Therefore most of your burn-in is going to be driven by bright content like photos or white text, not whether you're using 5% gray vs pure black.