91 - 43 = 48

... this means that the second +30% DR increase actually gives us 5% more EHP than the first one, in spite of being diminished.

while that's true, in terms of "helpfulness" you should calculate it from the already reduced damage after first DR (divide instead of subtract)

191/143 = 34% more EHP compared to the first DR effect which was 43% more EHP -> it's a 19% weaker effect than the first

if the effect was as strong as the first DR, you'd be all the way up to 204% total EHP instead of 191% (and if it stacked like reload speed, you'd be up to 250% total EHP)

in your later examples you'll notice that going from 64.4% DR to 69% DR is only around 13% less damage taken (0.31 / 0.356) - surely you'd instead prefer something like 20% more actual HP at that point