I'm still at my first company after switching career in to development.

It's nice here, and the people including management are great.

But I feel I should probably move on for the sake of my career progression, but I'm scared to leave and find myself somewhere worse.

Be very very sure of what this supposed "career progression" is before you leave something you enjoy.

If I hadn't moved to a new town, and if they hadn't also, I would have tracked down some of my old coworkers and asked them very nicely for a job. But we all 'graduated' and flew away.

I just worry that if I stay for years, and then for some reason do have to leave, it'll be harder to find a new job with my first as a dev having been at the same company for many years.

Generally I think long tenure is a positive signal, but you have to be growing. If you are growing and learning, and enjoying it, I recommend you continue to do so. Stop and smell the roses while you're at it.

In theory, skills are what matter, but I don't find that true in practice. Appearance matters more. A long tenure is a good look to most.