According to my expectations, resetting your computer app and data and reconnecting to your account should result in an more or less empty message history.
attendant3446
The last time I added a new device to Signal, I was given the option to sync old messages. Not all of them, but I think it was the last 60 days, which I think is enough.
kdmtctl
It does sync some timeframe in the recent versions.
nar001
It syncs everything that's on your phone, just not images and such older than 45 days I believe
According to my expectations, resetting your computer app and data and reconnecting to your account should result in an more or less empty message history.