Tejas Mk1A will defeat anything in Chinese arsenal in BVR combat. If it is offerred with Python V, it will have great WVR capability as well.
The Mk1A is supposed to fight an invader on our own soil. So it's gonna fight with the assistance of our own ISR complex alongside SAMs. Meaning, it's not meant to cross into enemy territory and fight on their terms like the more advanced jets will. Without offboard sensor support, it can't defeat threats like the J-20. On its own, it can only fight older jets on equal footing.
Also, the LCA Mk1A's job is not to fight and kill, it's to break enemy formations and send them home, forcing them to waste a sortie along with hours of preparation time. Once formations break, it takes time to regroup, giving enough time for the defenders to better prepare. So, when an enemy fighter sweep, meant to clear the airspace and make a path for strike jets, engages high endurance MKIs conducting CAP, the low endurance LCAs would be scrambled to outflank the fighter sweep and/or break the formation of the strike aircraft following the fighter sweep.
Otoh, when we are on the offensive, the MKIs perform fighter sweep, and the LCAs would escort our strike aircraft. And the LCAs role then is to prevent the enemy from breaking our formation, by, again, breaking their defensive formation.