@DanMills Do you have any other device mobile/pc etc. that was synced to your old Mac from before? Or did you just generate the synced phrase before resetting everything?
In the former’s case, you can use the sync phrase from that device (latest version) and use it on your new Mac. In the latter’s case, you would already have the 25-word sync phrase since you were on the latest version. So you would’ve to remove the 25th word from your code and add the last word as given here- https://alexeybarabash.github.io/25th-brave-sync-word/
More info here- Recovery key not accepted on fresh install - had to start from scratch! - #12 by Alexey