To be honest, having been in the exact same situation with other games, I think the problem is you.

I’ve played Warframe for hundreds of hours, I never once felt forced to spend money. I had fun with my friends, we were farming the Frames we wanted, at our pace.

While I get your point, I think you should really consider WHY you want to spend money. What’s you goal with the game ? Are you at least having fun playing it, or are you achieving a list of chores as fast as possible in order to feel the achievement of having 100% the game ? If it’s the later, it’s a never ending quest. There won’t be pride at the end, just frustration, and wasted money.