I have ~5 years as a FE dev. Has anyone successfully escaped it?
It worked for me. FE JS -> Fullstack JS -> Backend Node.js.
FE JS -> Fullstack JS -> Backend Node.js. -> Backend Python/Lua/C -> Desktop Software -> Embedded Software
It can take time, I was probably 2-4 years in each, required some study, and some interviews didn't work out for experience reasons, some did because I could prove I understood the issues at hand.
I'm now firmly in Desktop/Embedded which comes with its own challenges, but could not be happier about escaping JS and its... Oddness.
1. Relevant technical skills beyond front-end development.
2. Relevant credibility in those additional skills with your current or future employers.
Learning an adjacent technology within your current organization is therefore the easiest way to do this (see https://codewithoutrules.com/2016/04/27/which-technology/). Backend Node.js if you can manage it would be easiest, but backend anything will do.
And yeah, talking to a manager about a change is a good way to learn something new while being paid to do so.
But there are other ways, e.g. years ago I transitioned out web development and into distributed systems by becoming a contributor to an open source networking framework.
I think the answer largely lies in where you have the interest and willingness to learn new tech. Moving to full stack would probably be an easy transition, as knowing both front and back end is very marketable, and there's even stacks which require minimal retooling (read: node.js). Alternatively, there's things like ML/AI, vision, control, embedded systems, crypto, and quite a few other sub disciplines which you might find more interesting, at the cost of a steeper learning curve. MIT has quite a large number of courses online (even in video form) with OCW/edx, and there are certainly quite a few other resources available for learning if you have the time/interest/energy. Picking up new languages is probably a good place to start, and it becomes very easy once you've written a few thousand lines in a couple different languages since there's so much conceptual overlap.
In terms of experience/resume items in order to get hired in a different specialization, I think that small toy/fun projects that display knowledge in a given subject matter are very good in terms of marketing yourself. Of course, this route takes a fair amount of time, but can be very rewarding and interesting if you pick projects that align with what you find cool. What you choose to do doesn't even have to be novel/groundbreaking to be very rewarding. It's also imo easier to learn things if it's in the context of needing solutions to practical problems. If you do go this route, I highly recommend making a small website that showcases what you have done. I did this, and have heard first hand from several managers that it was a primary factor in me getting an offer. I've even bombed interviews but still gotten an offer because of my website.
On the bright side, I do think that front end is a very useful skill in general - I use my web dev skills to display my other work and occasionally to pay the bills. If you do decide to pick up a different specialty, your time thus far will not have been wasted.
Good luck!!