Doesn't achieving tenure mean research is optional but teaching is not?

From my experience, for all practical purposes, tenure does not change your job responsibilities. If you're expected to have a 4/4 load before then you are expected to have one after. Same if your expected to be research active with a 2/2 load.

What changes is the need to constantly renew your contract.

Much larger changes coincide with seniority changes (to full for example) or switching from being an adjunct.

Depends on the school

For some schools, tenure effectively means "all research all the time ... with some teaching on the side"

For others, it's "all teaching all the time ... with some research on the side"

Others will be a more-or-less even split

And it can depend on your specific department and/or specialty, too