Is it best to avoid lower body barbell based compound exercises and stick to isolation based exercises for day 4 of the cube method which is the bodybuilding day of the program?
It is best to avoid programs that you don’t want to follow.
Yes, the only barbell movement I would even consider is an overhead press and even then, lower intensity and higher volume. The amount of supplemental work perscribed (for cube kingpin) is already quite a bit of work for anyone that is even moderately strong, imo. Not to mention the assistance work on top of that.