With the exception of the ground combat overhaul, everything he's doing is fairly easy in context of the fact that he's literally rewriting it from more or less scratch anyways. If anything, this is probably one of the *best* times he could hope for to add or change things. Its not a matter of "not changing it, save five weeks", he's gonna spend 4 weeks writing X now, and then 3 weeks rewriting it two or three releases down the line, or 5 weeks writing X with the improvements he wants now.
If this was a thing that hadn't been running for years, if this was a thing that was paid for in any sense, I'd agree with you guys completely. But as it stands, I trust Steve will eventually release it, long cycles are somewhat the norm here for major releases anyhow. And he doesn't really have an obligation to get something out now, and make it better later, as a commercial or even crowdfunded project might. He can take as long as he wants.
I also wouldn't worry about mental exhaustion much. This is a hobby that clearly doesn't take up all his personal time, and he regularly works in spurts. We'll get a bunch of stuff over a week or two, then not much for a bit, then another little spurt. Sometimes he just takes a break for a month. He's been tinkering away at this for ten years, I don't think he's really at risk of overworking himself now all of a sudden just cuz of a release hype train kek. If anything, I'd say the recent surge of output circa ground units is just cuz "hey this is really fun to work on, and different from my normal stuff"