Quick update on my progress, had trouble getting 3DNUS to download 9.2 until I found out that it had been removed from Nintendo's servers, so had to get the full cia files from elsewhere. Then had to work out how to run sysupdater from the sysnand (some key combination in the gateway menu I didn't know about that runs gateway without emunand), then updated to 9.2 which was scary but went fine!
Then used the sliderhax browser exploit (was a little confused by what they meant by clicking the slider at first, because they suggest zooming in and of course the moment you do that it goes off the screen, it's in the top left for anyone who finds this thread and is equally confused) which worked first time, booted me to the homebrew launcher and then installed menuhax as in the gateway docs.
Then installed Ctrbootmanager but didn't realise it until I'd done it, I overwrote the boot.3dsx file with it, so I couldn't boot to the homebrew menu. Then I put back the one from the starter.zip but renamed it, and edited the ctrbootmanager config to set it as an option, at that point it started just booting into 9.2 sysnand and I seem to have lost my menuhax. So had to use the sliderhax exploit again (took a few goes to get it to work, as it kept crashing to the home screen).
I'll give the Ctrbootmanager another go later this week, but so far I'm pretty happy with the progress. Thank you again for the suggestions above!
I've managed to update to 9.2 and get it all going without losing my existing emunand which I'm happy about.