Thanks to everyone for sharing their thoughts.

Discussion of whether I should book single ticket or separate tickets is moot, because I actually cannot find this route anywhere on a single ticket. Neither OZ or TK website gives this route on their website. OZ's website is notorious for not offering any flights at all to cities that they do not fly themselves. North American airlines go the other extreme, that I can actually use the UA website and it can actually give routes that have no UA metal at all.

All this is theoretical, but probability of schedule change and reduction is becoming higher now. Just learned that LX and KE are reducing frequencies to IST as well. Anyway, I checked new prices and IST plane tickets do not cost that much, so in the worst case, I might just purchase another HAM-IST ticket and ignore the original one.