Making a stereo amp project (with repeating circuits and layouts)

When I do the ‘hundred’ annotation, the first sheet gets 100+, the next 200+, and so on.

For your case:
If they are all the same in eeschema (the same devices have all the same positions on the sheet) it should be safe to re-annotate all sheets with the 100-sheeme, as it will follow the same pattern on each sheet, so the numbers should come out similar.
I’m pretty sure you won’t be able to get sheet one to have R1, R2 and sheet two to field R101, R102 as it doesn’t work that way.

As for the reuse-of-blocks-of-layout see here (this one will be your best shot):