Yeah just looking at the gear and skills and sitting down in D3planner isn’t enough to calculate the max GR it can clear. It’s still useful for a lot of things, great for comparing different flavours of the same build, and can help you with order-of-magnitude kind of calculations. But too many things depends on execution inside the rift. Like, how many mobs can you hit at the same time with a sentry? How much squirts uptime do you have? How much value can you get of a condi? Can you tank a group of 3 elites while still dps:ing? How much value from zeis? How much do you gain from the grotesques exploding into eels that give you area damage? How much will teleport cooldown cut down your travel time between mobs? How much time do you need to pull mobs together? To drag elites? Exactly how good rift can you expect to get? Just the fact that experienced players can clear much higher than someone that’s relatively new is an indication that max GR potential is very difficult to calculate from first principle.
A better way is probably more data driven, where they look at how high builds actually do clear, as they do here: All Class Sets Ranked by Real Clears
Even there, I think they have an uncertainty of at least 3 GRs. Probably more. Clearly builds can be balanced much better than what they are now, they are spread over more than 15GRs now, but getting all within 1 GR is not feasible. If we get most within 5GRs, that would be an incredible achievement already.