I've got to remember not to include movie spoilers in this thread.
My thinking is that there was at some point in the draft of the story we see Tony enter "the garage" and we get a shot of a new Hall of armors. Tony knew the Mark 50 was no match for Thanos. It stands to reason he had a period where he was coming up with new designs.
As for the Mark 85 not being in any significant way more powerful than the Mark 50, the Mark 85 does seem more durable than the older version. I'd also say the Mark 50 was already just about as good a suit as Tony Stark could create given his level of science and technology. While he might be able to get a small increase in capabilities from model to model, there would be no revolutionary and exponential gains.
For example, the solid nano-material shield is replaced by a fashionable and functional energy shield.