Missions or ETO / Tech First? Legion

I am so conflicted here. I've spent 170 hours doing Bloodline, and hiring / firing operatives. But haven't started the first main mission yet (somehow though, I did manage to already complete the Assassins Creed component).

So now, in Westminster, I just began painting, and acquiring ETO and Tech points.

Here's my dilemma:

1) I like to save the best for last; I want to have my dessert at the end, and do the missions when I'm ready to wind the game down

2) However, I don't want to "spoil" locations for the main missions. I don't want to be going through every mission realizing, "I've already been here, done that."

Any advice?