RE: The (NEW) Beginner Guide to dCity Investment Game

Tech mining happens no matter the taxes, so the answer to your second question is yes.

If you are mining techs for selling the cards, GMO is the tier one tech with the highest price. If you are doing full tiers (1-5), its possible another tree could be more profitable, prices change over time - currently Advanced Management appears to be the hottest tier 5 tech.

But if you are just unlocking a single tier one tech to mine, GMO wins hands down.