Do I have to upgrade to Mojave from High Sierra if I am happy with it?

The basic answer is "no." Apple usually pushes out security updates for OS version two behind the current so, as long as the computer runs the software you need, you will be fine for a while.

You might ask in the Mac Pro forums if you can upgrade. Some Mac Pros with replacement Metal-compatible video cards video cards can run Mojave.

Mac Pro - Apple Community

To answer your question. No, you don't have to.

Allan Jones wrote:

A 2010 iMac will not run Mojave. Mojave requires a Late 2012 or newer iMac.

FWIW, the OP has a Mac Pro, which can run Mojave with the proper graphics card.

You're welcome.