Adding volume is generally a good thing, as long as your body can handle the TSS. The benefit of adding volume at endurance pace is that the TSS is low, while you still get the benefits of improving your aerobic base/fat metabolism.
One note: to get the most benefits from endurance rides, they are ideally long (greater than 2 hours) or fasted. This is to get the most benefit from training your body to use fat as fuel.
I’m currently in my base phase following (generally) a polarized approach. Next month, I plan to begin SSBLV2 plan, and will augment this with long outside rides at endurance pace. So similar to the approach you are thinking of.