I'm in the same boat, I have a 13 Pro Max, considering the 18 Pro Max, the main thing I'm not keen on with the 13PM is the poor screen brightness in sunlight, not sure that is a good enough reason to upgrade.Seeing people talking about coming from 15 & 16 models and here’s me thinking of moving from a 13 Pro. At least by leaving it that long I should notice more differences.
I'm upgrading my Series 5 watch to the 12 though, should be a big upgrade.