Bootstrap 3:-Most stable version

Bootstrap 4:- more responsiveness and components but not supported on IE9

Bootstrap 5:- latest version, uses vanilla instead of jQuery Not supported on IE11

My goal is to learn something that I am most likely to use in a real life project and also make it easier for me to transition to another version as the need requires

learn BOOTSTRAP5 it has new features like offcanvas and so on

How about you just learn "bootstrap" rather than just put your focus on it's versions.
Learn any UI framework/utilities you can grasp.
scss, sass, tailwind, material... anything bro.
Your focus should just be, knowing the bare minimum you need for a fully functional project.

You project/portfolio is a proof of your knowledge.

The first time I used Silverstripe, that's still the last time I used that CMS. I didn't decline, rather I took a 5hour crash course and bam. I was modifying that CMS like say na my papa do am.

For the fact I knew basic php, js and css.
(WP, Magento, codeigniter, laravel and the likes...)
I just knew it's all about syntax and paradigm for each and every one of them.

Learn CSS very well. You can pick up any CSS framework in a day max.