Trading off resolution vs quality
Decreasing the size a little won't help much, decreasing it a bit more induces a fair bit of aliasing (unless it smooths it also) butchering the quality without much gained.

Cutting the size 50% will reduce errors and double the jaggyness and be easily enough to save a lot of space, question is: Do you want 360P Video? You really wouldn't want any smaller.

So while that is one way to save space for Archiving I would recommend using 'Q' at 40 to test one file at 720P (output) and see if you can tolerate the quality.

H.265 has superior output quality as well as an approx. 50% reduction. Decreasing the resolution in half means an approx. 25% reduction alone, along with approx. 50% from H.265 your going to be under 15% WITH the same quality.

So decreasing the resolution from 720P will be either too drastic or induce greater errors in the output (it will begin to look like mush). Best to try using 'Q' alone as see where between 30 and 40 you can stand.

Here's an example from one particular Video YMMV.



Note that going from 24 to 40 gets you down to 10% (in that one example).