Every aerospace industry is in a constant learning curve. At no point can one reach the pinnacle of development their is always new tech and improvements to be made.
The reason they brought the su-35 to gain information on the 117S engine and experience with a super maneuverable thrust vectoring aircraft which they eventually wish to apply in j-16 and j-20.
You cannot compare tejas and AMCA to their chinese counterparts because both j-11 and j-20 are already inducted in good numbers with ongoing serial production while tejas in our case has only recently begun proper production.
I would just like to point out one thing: A lot of the assumptions that you are making, such as the SU-35 was bought only to study the engines, are CCP propaganda talking points, and not the truth. You cannot just reverse engineer engines by looking at them, it's not that simple.
The CCP military most likely bought it to study the SU-35 radar, sensors and avionics in order to copy them and their modes/programming. However the Russians are not stupid, and only gave them severely downgraded versions of the avionics/sensors. Don't go by what is advertised about their super radars and extra T/R modules in CCP military propaganda brochures. If the CCP military was so advanced, why did they buy the S-400 which contains 2 decades old (severely downgraded for export to CCP military) Russian radar technology, which is another 2 decades behind modern western radar technology? CCP's game is to "fake it till you make it." They always overstate their capabilities. Their access to western technology through Israel and computer hacks, etc is also limited, and not as great as they would like you to think. They are good at making it look good at air shows, but in terms of capabilities, CCP military radar/sensor/ECM technology is still far behind the west.
PS, their J-20 is a failure in terms of achieving decent RCS reduction (due to faulty design, less reliable RAM and the radar/radome still being exposed to modern enemy radars), which is why it has been repeatedly tracked by IAF and Taiwanese radars, and its avionics/sensors are also quite outdated and less reliable than what they advertise; but you wouldn't know that from what is advertised in their propaganda brochures. The engines used on them are weak (giving it a crappy thrust to weight ratio) and lack reliability, even more so than the unreliable Russian engines that they are copied from; and J-20s are struggling to operate from high altitude airfields.