Is there a way to remove a menu item from the active trail without removing it from the menu parent?
This is a common headache. I keep meaning to write a contrib module to address this by letting users define a "canonical" menu item in the event there are several menu items that point to the same page in the same menu.

That said, there are some workarounds you can use.

The best option is to use the Redirect module. You create a redirect to your Donate and Volunteer pages then put the redirects in the menus.

So if your volunteer page is at /volunteer and donate is at /donate, then you can create two redirects. One called /go-volunteer and another called /go-donate which redirect to their respective pages. Then you'd put /go-volunteer and /go-donate in the menu which, when clicked, will take the user to the appropriate page.

Since they're redirects, they won't trigger the active trail.