Very cool. It's fun going back and updating after you've gotten to use it for a while. You know what works and what doesn't. Very satisfying when it's complete.
Definitely time to fix it. I have had better luck with slide vents as the butterfly vents always seem to rust up or warp and don't want to operate smoothly. Just my opinion....
I like the idea of being able to push the ash out without a lip at the bottom but that lip will be nice to ensure a solid seal for your door. Maybe time for an ash pan that you can just pick up/out and dump?
I'll give the butterfly vent a try, I like how they look.
Ash pan on the circular FB will be fun, I'll have to use thin gauge steel like car body steel and panel beat one to shape. Just had a different idea of leaving the lip to seal as you say, but fitting a "ramp" inside to sweep out.
Oh yeah, forgot about the vent lock effort. As it's not a balanced butterfly vent it would always drop to the lowest point, so welded a bolt through the butterfly, then made a screw lock with a nut welded inside a bit of tubing blanked off each end, and holes drilled for a length of rod.
Half a turn and it slides free, half back and it's locked in any position you like / need.