Is there any advice which you used to give to conlangers but now
consider misguided? What was it, why did you think it was good advice,
and how has your attitude changed to make it not-good?
This is going to be pretty boring, but: nah, not really. My stuff is mostly not advice per se; it’s just introducing linguistics to people. When I do have regrets, it’s usually that I haven’t covered somnething, and the solution is usually to write another book. 🙂 So a lot of things that didn’t get into the LCK got into ALC instead.
I did take the opportunity to revise the LCK to give a better introduction to aspect, though.
I always wanted to give a better overview of transformations and how they revolutionized syntax. I studied that a lot in college and found it fascinating. On the other hand… well, I can’t really say a conlanger has to know that stuff. Plus the field never reached a consensus on the best way to handle syntax. (My Axunašin grammar attempts to do justice to transformations, though I think it’d have to be three times as long, and include lots of cumbersome trees, to really explain the concept.)