Make offshore mines better in the lategame

They become too inferior to contracts, some even to just island production, some options to buff them are:
1) Upgrades on tech tree, that will increase their efficiency
2) Edicts, that will cut one of the costs (unity,workers,maintenance) at the expence of other 2, or at the expence of the higher tier maintenance
3) Split cost of the mine on per month and per shift costs - just like contracts work
4) Just make each consecutive shift cheaper, at least in unity
5) Add really far away mines, that are hard to get too, but straight up more efficient than counterparts
6) Or maybe you can deliver different resources to your mines (food, household goods for example), so people working there will be happier and take less unity