One GNOME extension I forgot to add to the list of stuff I now use is “Rounded Window corners”. It rounds every bottom corner of every window, so even older GTK3 apps, and non GNOME apps look a bit more in line with libadwaita apps.

Also, I added the GNOME thunderbird theme, and it makes a huge difference in how Thunderbird integrates visually with the rest of the desktop. Of course, it might break with every update, so we’ll see.