The value of Trade with any Nation (Minor or Major) can help improve the Relationship. So if you consume what the Minor Nation is producing--- and keep trying to buy all of it--- then that will shut out other Powers from building a relationship with that Minor Nation (for a while). I concentrate on two Minors early and try to purchase what they are producing. Once you have an Embassy (which will also improve the attitude of the minor nation) a couple $1000 grants should put you over the top. Also, keep your subsidy the lowest for the minor nation, it will encourage them to trade with you. (Hold down the Control button when you set subsidies. The game will show you the lowest subsidy for that Minor nation.)

If you have a Rival that is a big trading nation, they get developers quickly and will use the developer to take control of assets in your minor nation early so as to screw you. I haven't found a cure for this other than to delight when I take that S&*D&S Capital.

Thank you so much for this, i finally got one lol, again thank you so much, made this so much easier

If you manage to turn the minor nation into a colony, you can apply a colony boycott to the offending major power and watch him lose all his investments. You can then buy those resource tiles for yourself. (you keep any improvements built by other major powers)

I don't ever saved a minor where another Power is purchasing assets. But what you suggest would be fun