So, turns out I'd forgotten to swap out import 'gsap'; in one of the files, which explains why the size didn't change and everything worked immediately.
Once I fixed that, I did have to make an explicit reference to CSSPlugin for everything to work (importing from /all or /TweenLite made no difference, except I had to manually import some eases from EasePack when I wasn't using /all).
I gained 18 KiB (going from 800 to 782KiB minified, including all other assets), which I think isn't really worth the added maintenance and potential for invisible mistakes. (If you don't import CSSPlugin, there's no warning at all, but the tween just don't do anything – I'm not sure that there isn't something else silently failing somewhere else.)
It makes sense that the gain is so small, I'm using TimelineMax so the only thing that isn't used is the added stuff from TweenMax. It's probably possible to fragment GSAP into more granular chunks, but that's a challenge for another day, haha.
edit: maybe, when transitioning to 2.0, you should consider renaming TweenLite and TweenMax to just Tween, and TimelineLite/Max to just Timeline. The functionality would be defined by what you import anyway – I don't think there's a reason to have both names, is there? It can be a bit confusing when starting out with GSAP to decide whether to use Lite or Max versions, when, really, as long as you need the Max features once, you might as well just use it everywhere.