Also a long time user. Will add my opinion to your points along with a few extra nitpicks.
Overall
Agree that they’ve done an amazing job. Thanks so much to the team.
Headings
The fact that the markdown is finally hidden is one of the things I’m most excited about. But when highlighting it would be great to see an h1-6 icon instead of the three lines.
Info panel
Agree with almost everything you’ve said. For me, having a dedicated side bar with the TOC visible would be a game changer. A collapse-all feature would be awesome for the longer notes with many headings. Ulysses is a nice example of an app that implements this well.
Note list
Agree about the ordering. Pins only provide one level. Being able to manually order both tags and notes would be really nice. I can finally stop adding Greek letters haha.
Would also be nice to hide the last-edited date, it has never been relevant to me. Just a clean list of titles is the dream 
Themes
What I would love so much more than extra themes is simply the option to create my own. Even if it was just selecting the hue for the main/accent colors. Duotone Snow is the GOAT, but I don’t need that red accent everywhere haha.
Personal opinions
The new tags stand out unnecessarily. Preferred a consistent text color and subtle highlight.
The link-editing icon was cleaner and more balanced as a simple chain. Don’t need that wispy little line 
Again, great job to everyone involved.