Longtime AA loyalist here looking to diversify my miles & points earnings, so am curious about consumer sentiment among those who have been in the primary transferrable systems for much longer than I have - focusing mostly on AmEx, Chase, and C1 (unless there are any strong Citi advocates out there?).

My situation:
AA Executive Platinum (Oneworld Emerald) for 7 years running. Take between 40-70 flights per year for work, earning around 300K Loyalty Points with AA each year and around 400K AAdvantage miles, mostly through flying and booking my hotels using the AA portal. I use my miles for premium cabin redemptions on Oneworld carriers. Though AAdvantage miles are still one of the most valuable currencies out there, the holy-grail redemptions are less common and I want to maintain a cache of transferable points. I do not plan to transfer points to hotel currency, as I find much greater value for premium cabin redemptions.

I use my AAdvantage Citi Executive World Elite Mastercard (Admirals Club access, earning 1 LP and 4x miles/dollar on flights and 10x miles/dollar on hotels booked through aadvantagehotels.com), until I requalify for EP status, at which point I want to move my spend elsewhere to capitalize on better category bonuses. I generally requalify about midway through the qualification period, leaving roughly 6 months where no spending will go on my AAdvantage Citi card.

My plan:
I've added the AmEx Platinum to my wallet to try out the tried-and-true AmEx ecosystem. I'll easily hit the SUB and enjoy the 5x earnings on flights, which I identify as the best perk of the card. The extensive lounge network is nice for when I'm not flying AA or at an outstation, and especially like the Sky Club access when flying DL. I'll use the hotel, airline fee, Uber and Clear credits, but time will tell whether I really value those benefits.

I'm considering adding either CSR or VentureX for comparison's sake to see which program suits me best. I may try them both over the next year or two. The longer-term plan will be to maintain probably 2-3 "Premium" travel cards - the AAdvantage Citi card might remain one of them, or I may ditch the cobrand in favor of something else. Doing so might drop me down from Executive Platinum to Platinum Pro, which is still Oneworld Emerald so not really a big deal.

My questions:
What program(s) work best for you and why?
Do you hold or pursue status with an airline, or are you a free agent?
Do you hold multiple premium cards (say among AmEx Platinum, CSR, VX), and feel that you get adequate incremental value from each one, or do you stack cards within one ecosystem (i.e., AmEx Platinum and Gold)? If so, how do you split your spend?
If you've held 2 or 3, which do you feel provides the best lounge experience?

Thanks for your input here!