bolded part is relevant...if you're lifting with an active intention of getting bigger, you're chasing a moving target, so no size will be right for you both now and then. further, figuring out what'll fit you a year from now is pretty close to impossible, so you have a couple options:
(1) choose a fit that works for you now and upgrade (or not) when they no longer fit
(2) hold off and size/buy when you expect your size to say roughly the same
there's no way to accommodate both your current size and where you'll be in the future (assuming you do some serious work) with a single pair of jeans. should be pretty obvious, this isn't magic.
Correct. And, if the jeans do have decent stretch to them in the long run, then fitting now + stretch = fitting then.
Buy what fits.