Just did a roundtrip from the US to IST early this month. OS/LH/AC will have you transiting through airports that will require quarantining on the way back (if this is round trip.)
TK is great as you avoid a 3rd transit country (and their safety protocols have been solid), but their service has been significantly downgraded and their schedules have been a mess. I was going to fly TK, but gave up and cancelled the fourth time I had to call in due to flight changes/cancellations in a single week. While I was trying to stick to Star Alliance to burn some miles, we ended up booking with Qatar. Service was great (only minor alterations from normal in all cabins, we flew Y there and J back), safety protocols were great and you are allowed to transit Qatar without a quarantine in the US upon return. Flights to and from the US we pretty empty...enough that everyone got 2-3 rows in a section to themselves in Y. Qatar to Istanbul flights were more full. They require a mask and face shield in Y, but you are not required to wear them in J while you are in your seat. With frequencies to and from Turkey down, we did need an overnight layover both directions.