mouse batteries don't last

Thank you. I feel this is the most likely cause of your battery drain:

That model, discontinued in 2017, seems to have a habit of the sliding cover that doubles as a power switch not contacting the actual switch—under the battery cover—once a flat metal spring inside gets "old and tired." Thus when you think the mouse is turned "off," it is not. The easy at-home fix is to simply adjust the spring to restore the original degree of contact.

Also check that area for a build-up of dirt, apparently a common cause of power issues in that model.

We are not allowed to post links to non-Apple repair sites, but you can easily find illustrated instructions for this five-minute fix on the web by using the search string "fix Apple mighty mouse power button.

Apple has several battery-powered mice. Which to you have?

Also with what Mac model (and OS version) is the mouse being used.