I ran into a similar issue, I have a zcashd wallet I created around NU5 that I haven’t been able to access because every time I would start zcashd, it would trigger a full re-scan.
I decided to recover it in Zenith with the seed phrase and the exercise pointed out a number of places where the wallet scanning could be optimized, we’ll be releasing a new version with these optimizations once the changes are complete. We want to make sure people can use their wallets for years and recover them if needed.