You can't get Handicap bonus in Custom Races because the money rewards are already pre-determined at the race setup. And, on the average, the game awards more money in GT League events anyway
Custom Races can be an answer tho if you are looking for a very long single grind (the "Leave the race for a night with buttons/triggers held down by rubberbands" type of grind). If you get the gamepad setup right, you can get couple of mil overnight. But it can have a negative effect on your PS4's longetivity and your electricity costs
The most money you can get in Custom Race is around 7-8 million Cr., which can be accumulated with couple of hours of BMB/Nostalgic LeMans grind, so it's up to you to decide which one is more effective
Although, if you don't have enough time to play the game and just get in everyday to complete the Daily Workout, this setup can be useful for you
Car:
Pick a light car which can be upgraded to max N200 (i use the classic Alpine A110)
Lower the weight further, if you need
Tune the gearbox so it can hit 315 km/h
Race:
Special Stage Route X
2 laps
Full grid, start last
No Boost
Gr.2 opponents
Professional difficulty
They'll pull off at the beginning of the race, but you'll easily catch up to them by the first banked corner and overtake them on second straight (just make sure the car hits 310-315 km/h)
Extremely easy 150k Cr. with Clean Race bonus and completed Daily Workout routine