At the v5 of WordPress, is it worth the effort to learn how to develop a theme for it? (Only for blogging purposes)

Depends, do you use it or do your customers use it? Then yes

Otherwise hell no, run while its still possible.

@Codex404 I'm used to write the HTML (actually using Jekyll for modularisation/layering) but tried it to host on my server WordPress, the experience is quite user friendly but I fear the amount of work needed to make something custom because of my custom style and blog structuring.

If possible avoid bootstrap. It is styling elements which basically forces you to add classes to everything to reset it.

Took me four hours to fucking overwrite a padding bottom of 8px on one single label element. It was used within an angular component and it made vertically centering impossible untill I added a margintop of 8 pixels to a container.

@Codex404 lol, that's a hassle to deal with, anyway I avoid bootstrap for personal staff, it's too genetic.
Maximum I use it for quick demos or prototypes. u_u