There seems to be a non-avoidable problem with your current theme in that some apps will just like odd and ugly due to the limitations of the toolkit. Especially buttons.
On the other hand, Ubuntu's Yaru Gtk3 theme (for example Yaru-blue) manages to make older apps look more modern without UI issues.
I was wondering if you would create a variant of your theme with the design idea "Looks good alongside Adwaita Gtk theme, but doesn't attempt to replicate every aspect of the new theme"?