Stay with the mini bands for bench. You might be able to get away with the light band for squat but you wouldn't be able to put very much weight on the bar.
If you like the routine you should definitely invest in some chains.
And definitely don't use bands more than 3 weeks or so. They make you incredibly sore. Just got started on bands for this cycle
and after Sunday's workout I'm still real sore today.
Try the eight week cycle
to start with. Max out or go to a contest at the end of that cycle
then start again using new maxes.