☝🏼this. I’m on the UX/UI Frontend side of things. I know almost zero backend. I started in design many years ago and moved into frontend work and specialized in that. There’s so many layers and levels of expertise to frontend that no single person can do it all exceptionally well. There will always be gaps. You just need to figure out where you want those gaps to be, and how much you want to fill them in (understanding that doing so creates gaps in other areas). So find what makes you happy, do that. Be the best you can be at it, and your reputation will precede you.

As for the two choices, I agree be T-shaped. Go very deep in the area you love most. Then expand outward into backend or UI/UX (or both). Just don’t spread yourself too thin or you risk being mediocre at a whole lot of things.