I think if you’re having to grind away at 50 rpm then you’re likely not targetting what the workout was trying to achieve. I sometimes do the sweetspot workouts outside. I usually do a big long flat ride and use my power meter to roughly do the same power as the workout. I keep the durations the same.
My suggestion is important workouts like interval workouts are best done inside. Lower intensity rides and especially the easy rides can be done outside. With these when I do them outside I don’t usually bother too much with the power. I just use the power meter to make sure I don’t go too hard plus I’ve done them often enough inside and know what my heart rate should be so I try to maintain the same heart rate.