Flexx VS DearPyGui

Why tk then? Maybe look at https://github.com/flexxui/flexx or a python Electron wrapper.

Flexx transpiles python to js

Unfortunately no. At least not a designer. If you want to create web GUIs with python you may have a look at https://github.com/flexxui/flexx

For native GUI, DearPyGui[0] as modern as you can.

For browser web-based GUI, you can use nicegui[1]

[0] -- https://github.com/hoffstadt/DearPyGui

[1] -- https://github.com/zauberzeug/nicegui

If it's just to align text in the centre or to the right of a box, you could use a button and, by using theming, hide the fact that it's a button. https://github.com/hoffstadt/DearPyGui/issues/1111

Personally, i like PyQt or PySide they have basically everything you need to make a proper app or even simple apps, but there is another DearPyGui this is basically python version of imgui the c++ library, it provides some good customization options and can be used for simple or complex projects too, their discord is also very helpful.

There are some examples in the showcase as well. https://github.com/hoffstadt/DearPyGui/wiki/Dear-PyGui-Showcase

I'm a big fan of DearPyGUI

Use a button and set width of the button equal to the width of the window. Using theming to set the button text alignment to center. A comparable solution, for aligning to the right, can be found here. https://github.com/hoffstadt/DearPyGui/issues/1111

I was using PyQt5/PySide which will give you a nice native look but I eventually shifted over to DearPyGui for most of my smaller utilities. Its just faster for me. You will get more options and freedom with PyQt/PySide though and can use QT Designer to design your interfaces pretty easily.