Everything below is personal opinion.

Personally, I do not believe you should tightly couple front & back ends, or particular frameworks (both front & back-end).

Learn the underlying language(s) and use modern practices with them. This is much more important than learning a framework, as the framework will eventually be something you utilize instead of being a crutch. Find a local developer group/slack/discord and ask for suggestions on where to learn from. Pay more attention to official language documentation than stackoverflow when looking for language-level answers.

Go will likely get you better pay than PHP, but PHP will be much easier to get up and running with. Security is not tightly coupled to anything. You just need to know how to do things well... you can code crap in any language.

Unless you want to be a poor soul being wrung out by a startup or dev shop, I would not try for "full stack" immediately. Get good at one thing and expand out from there. You will likely learn some bits of the other side as you grow.

The market is flooded with JS devs from bootcamps (I do not believe the pandemic has changed that), so I would say to try your hand as a back-end developer. Of course, if you feel you are more of a visual person, you should stick with front-end work since you will be happier there.