ajc - thank you for the suggestions (sorry for the very late acknowledgement!). RS Migration Tool wasn't an option as we were going Native -> Native.
I ended up doing an in-place upgrade after a bit of graft on the server. For anyone searching this in the same scenario these steps worked for me after trial and error (we have HEAT reporting configured for Custom authentication and we don't use the OOTB reports).
> back up and comment out the 4 x SaaS extensions in RSReportServer.config to allow RS to upgrade (will show as a blocker in Upgrade Advisor),
> upgrade SQL/RS/AS/IS,
> uninstall/reinstall HEAT reporting
> manually correct the instance name in rssrvpolicy.config, rsmgrpolicy.config
> manually insert the Security and Authentication SaaS extensions from first step
> rerun configuration wizard(s)