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.
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.
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.