Tk and Ttk are essentially different toolkits with a similar API. There are some decent looking themes (ark, breeze, yaru, there's even a bootstrap port) and also some native themes for Ttk. I believe the native Windows theme is Win32. You likely don't want an UI backed by Win32 widgets, for various reasons (poor performance, looks kind of bad, flicker/redraw issues, abandoned by MS).
I feel like desktop UIs on Windows have been in the state of "It hurts when I do this" - "Stop doing it then" for a number of years. "Just put everything in a web view" might actually be the best way to have a GUI on Windows nowadays, if you really need one, and otherwise you simply avoid having to run an application on Windows entirely whenever you can (=pure web app).
I feel like desktop UIs on Windows have been in the state of "It hurts when I do this" - "Stop doing it then" for a number of years. "Just put everything in a web view" might actually be the best way to have a GUI on Windows nowadays, if you really need one, and otherwise you simply avoid having to run an application on Windows entirely whenever you can (=pure web app).