Just completed the spend on my first miles credit card and now I have a new goal: Earn enough miles for two roundtrip tickets between US and Germany for either Business or First Class seats.

I know that points earned on cards such as American Express Membership Points, Ultimate Rewards, and SPG Points all transfer 1:1 with certain airlines (and I see some even have a 5,000 mile bonus per 20,000 miles transferred). My worry is that I will end up with a bunch of cards with, say, a 40,000 or 50,000 sign-up/spend bonus that I won't be able to combine for one specific trip. I need to earn a bunch of miles that I can use in coordination with the others - if that makes sense.

Focusing on just one person, it looks like I need around 135,000 miles to fly roundtrip in First Class. What combination of cards should I focus on in the next year or so in order to earn that many points/miles that I will be able to apply towards a single roundtrip ticket purchase?

And, depending on the cards I apply for, do I need to be concerned with point/mile expiration? Or does an occasional credit card transaction generally keep my points alive?