* Working with users & designers, rapidly iterating UIs on some bleeding-edge messy ecosystem (React, etc.), dealing with whitespace and color palettes, state management and associated race conditions, and having clearly visible creative output at the end to show for it?
or...
* Working behind the scenes with other techies, vendors, specs and docs, architecting against best (or at least "known good") practices & patterns, enforcing rigidity and consistency and good versioning throughout your APIs, optimizing the hell out of every query, containerizing everything and jumping in and out of DevOps every hour, getting to learn various platforms and stacks in depth in order to produce a stable HTTP endpoint, working with many clouds... all to produce a remarkably well-engineered response that fellow engineers could appreciate but no user will ever see?
You can get a pretty deep, but different, sense of satisfaction from either calling. And these days there is really no end to the depth of specialization you can achieve, only the breadth. Even within "frontend" and "backend" there is already several lifetimes of learning, and the pace of development is only getting faster.
The frontend tends to iterate quicker, fast and furious, while the backend focuses on stability and resilience. The engineering cultures can be different (though not incompatible). In general I think the backend stuff is "harder" in that it's more actual engineering, and tends to get paid better. The frontend stuff is difficult in a different way -- creative and visual, often -- which for some reason companies don't tend to reward as highly. In our culture, engineering is thought of as more difficult than design, unless you're Steve Jobs or similar.
But at the end of the day, either path (or remaining full stack) would easily get you a much-more-than-livable wage at any decent company, so it comes down to things like work-life balance (do you want to be on pagerduty?) and just how much you enjoy each bucket of work, and the kind of people you end up working with.
If you're not sure, don't decide after 3.5 years. Maybe try a FE specialization for a year or two, a BE one for another year or two, etc. until you find a niche you really enjoy.
For me it took like 15+ years to realize that what I loved was being able to quickly develop UX with an abstracted/serverless backend, using Next.js. But others hate that loss of control and want to work on the parts of the stack that I'm grateful to never have to touch again. YMMV... take time to explore!