Based on KF-21 project, I think joint development is impossible, so is the F-35,but somehow I think new F-16 blk-70s or at least upgrade kits are possible ?
To be honest,I don’t get the obsession with F-35 ,it’s not like there’s going to be a war with Greece ,projects like TF-2000&altay could use more funds ,let alone the drone industry ,and they would actually help building an indigenous defense industry,why wasting money on a fighter jets?
Well they are rumors for now, but there might be several reasons for a Turk-US rapproachement. The biggest is the growing threat of Russia. Another reason is that Turkey is developing various weapons that compete or will compete with US weapons. In this case it is better for the US to keep Turkey reliant on its (sub)systems like the F-35 and the F404/F110 engines. The various Turkish ships also need GE's gasturbines and TEI has recently signed a deal with GE to co-produce the LM2500.
As for the need for the F-35, the TF-X will only arrive in sufficient enough numbers to be advantageous for Turkey, not earlier than 10 years form now if everything goes right. This gap of 10 years might be plugged somewhat by a supersonic MIUS and a whatever stealth drone TAI comes up with and potential a combat version of the Hurjet, but it is better not to bet on it as this is the first time Turkey is making such niche and high tech products. F-35 gives Turkey room to breathe. This doesn't mean it will cut R&D funding in favor of foreign systems.
The US might also be eyeing the use of the TCG Anadolu in the black sea under NATO command. F-35B's will thus come in handy for not only Turkey, but the entirety of NATO.
For now, both NATO(+US) and Turkey are codependant, whether they like it or not.
As for the funds and timeline of various systems, every project has a schedule and budget. The Altay is ready but it's waiting for a powerpack. Soon the Altay can choose between three powerpacks, two indigenous (one diesel, one hybrid/electric) and one foreign(South-Korea). Throwing money at it will do little wrt speed, if any.