Is there any way to connect/ synchronize Litchi app to DIY drones?
Currently I would say NO since Litchi’s current list only includes DJI Drone models and I have not read anything that would indicate that they do. Here is the list of the Drones it does support. Litchi is compatible with: Mini 2, Mini SE, Air 2S, Mavic Mini 1, Mavic Air 2, Mavic 2 (Zoom/Pro), Mavic (Air/Pro), Phantom 4 (Standard/Advanced/Pro/ProV2), Phantom 3 (Standard/4K/Advanced/Professional), Inspire 1 (X3/Z3/Pro/RAW), Inspire 2 and Spark.
Ardupilot and Inav, both of which are free open-source downloads, seem to be the auto-flight choice of most DIY drone owners.