@andyrose I also use a FocusRight Scarlett: https://www.amazon.de/gp/product/B07QR6Z1JB/
I am very happy with it.
It came together with the software Ableton Live 10 Lite.
I record the video with iPhone12mini, record the sound with the Ukulele plucked into the FocusRight Scarlett, that records into the Ableton Live software on my iMac.
Then I export the audio file from Ableton.
In iMovie I import the audio file and the iPhone video and replace the iPhone’s audio line with the one from the Ableton export.
When I play the non-electric Uke I have the same setup but pluck in a condensation microphone into the Scarlett instead. I use this one: https://www.amazon.de/gp/product/B089SX1DX2/
I hope this helps,
Andrea