Alternative way for truing disc brake rotor after a couple of truing procedures
Your best and safest option is to replace the rotor with a new one.

Bend metal enough and it will deform permanently.  

In addition, rotors do wear away slowly over time.  There's a chance yours is getting thinner, and more vulnerable to bending.   Generally they're ~2 mm thick when new and unservicable at 1.6 mm thick.

Finally, if your rotor has overheated at any time (IE its blue or rainbow-coloured) then the rotor has also been weakened and is vulnerable to bending and warping.

Brake systems are the one part on which you don't want to cheap-out.