Yes, works on my phone for about 16 narrow lines, but if they are longer lines wrapping across multiple lines, it will stretch way off the display - and still not show the "See More". Appears a character count would work better there than a height.

Wrapped lines are fine. It counts content height so if it wraps that will be considered against the height. The only reason I can see this being a problem is if the width of the activity container was changed after the activity was loaded as then you will see problems like content being too large. At any rate have added a feature ticket to CB to see if our cbmoreless implementation can be improved to be better overridden by CSS. As currently it can, but not if something has set a height in the cbmoreless settings.

forge.joomlapolis.com/issues/7986

( I have no problem modifying the JS knowing I will have to fix on every update )

It's not a matter of modifying a simple line of JS. CB Activity JS is very complex and large. That's not a possible solution here.