Selected categories or brands first, the rest once you see that it works.
A bike shop sells everything from complete bikes to bolts and gaskets. Lifting that entire range online in one sweep rarely turns out well, neither for you nor for the customer trying to find the right part.
So it works just as well to begin with a defined slice. A couple of categories that sell, a brand you are strong in, or whatever is easiest to describe and ship.
The rest of the range stays in the shop exactly as before. The web shows only the part you chose, and once that part works, we widen it.
The selection is yours to make, and it can be cut in several ways.
The point of starting small is that you see how data, images and orders behave before the whole register is out there.
All you have to do is point out where you want to begin. The connection is built to take more, so expanding later is not a new project.
Start where you are strongest and grow from there.