1. Yes, J is not bad. I'm sure the product on TK25 TPE-IST is lie-flat (and not flat bed- and I believe there are variations on the 343) but TK34 from IST-IST is on the 77W and will have fully flat J.

2. I've flown TK both Asia-Europe and Europe-US in J and catering is great. Do&Co is amazing. I seemed to have meals with a bit of a Mediterranean flair to them- think along the lines of meat and spices but quality was good.

3. You might want to try both their arrival and departure lounges, both are great and accessible by J pax. 8 hours is enough time to enter the city but the lounges are top-notch and can occupy your time.

4. Service in my experience has been great.

5. In general I've read that you're sometimes delayed out of IST but shouldn't be a big deal. TK should be as safe as any other big airline out there.