1. Full pre-COVID service is more or less restored
2. IST operates as normal - most TK flights are more or less on time, if you commence your trips in AMS book longer connections - you will most likely face more issues in AMS than in IST
3. TK offers all published benefits - but nothing more - no extra recognition. Upgrade vouchers are only for E+, OPUP's are prioritized based on FF status
4. I would never consider ST as they do not offer lounge access on wholly domestic itineraries. I sometimes travel extensively within the US and my TK*G gives me access to UA Clubs, while my AFKL Gold or Platinum would not give me access to DL Sky Clubs. My not be your case, but for this reason I treat ST as an inferior alliance...