After studying my heart rate (which can vary a lot depending on time of day I work out etc, food intake, how warm it is) I tend to focus on how close I feel to the limit.
So I can of judge of far off my limit I am, and kind of go from there.
Just because a workout is hard doesn’t mean that its a 8 out of 10, might be a 6 even though its super tough. Same thing with a long endurance ride at high zone 2/low zone 3, it might also be a 6, but a different kind of 6, if that makes sense?
RPE, similar to a pain scale, is particular to a person. What matters is that you are consistent in assessing with yourself not with others. If it feels like a 10 to you then its a 10. It might be an 8 for your buddy, or a 6 for someone else. Doesn’t matter.