I have never actually done any specific WordPress training so it is hard for me to make a recommendation on this but as WordPress operates as a PHP framework I find having a good understanding of HTML/CSS and PHP is the way to go for getting stuck in with plugin and theme development.
Once you have an understanding of Object Orientated PHP, diving into the WordPress docs seems to make a lot more sense and there are lots of guides on creating themes/plugins once you have the PHP understanding.
Although these resources are more geared around learning to develop in the Laravel framework I find that the first steps on this site are the exact same as I would proscribe for learning WordPress development and address the basics of writing HTML/CSS before progressing onto PHP: https://onramp.dev/en/modules This project is open source and was created to help onboard people into the development industry, almost all resources/links are free from what I can see.
Once worked through the resources and get to the “basic Laravel” stage, you could instead progress to looking at the WordPress codex which has extensive documentation on creating themes and plugins.
Hope this helps!