This is an impossible issue for me to tackle.
Since it's two-way role syncing one of the ways has to be prioritized.
In your case, you want the discord roles to be the priority.
However, here is where the issue stands.
When a user joins a server, it compares their oxide and discord roles. If they don't have a group in oxide, but have it in discord, the bot will remove it from them in discord. If they have a role in oxide but not in discord it will add it to them in discord.
Unfortunately, I cannot do that both ways because the bot does not know if the user is supposed to have the role or not.