Upgrading from macOS Big Sur 11.7.11 to macOS Tahoe on iMac

That depends on the model of Mac you have, not the version of macOS it is running.

You need a 2020 iMac or later to upgrade to Tahoe.

If you have an Apple Silicon iMac from 2021 or later (as your device line suggests) the upgrade to Tahoe should be available in System Preferences ➜ General ➜ Software Update.

click here ➜ Update macOS on Mac - Apple Support

Identify your iMac model - Apple Support

if it was me, and if you have the time, I would upgrade one macOS at a time. while it is technically OK to skip over a macOS, I have seen times where skipping over a macOS causes issues. and at very least, I would upgrade to Ventura before trying to get to Tahoe. there were a lot of changes in Ventura as compared to Big Sur.

you can find the downloads for the other macOS here >> How to download and install macOS - Apple Support

Thank you jeffreythefrog, been through a heap of messages on this, but your suggestion of one at a time seems the one most sensible

you're welcome. :)