It shouldn't be a problem to take it back and exchange for a 40A. If you put in a 100A breaker you will need to feed out with at least 100A wire. The breaker's job is to protect the wires running through the walls/conduits from catching fire. Whatever size breaker you put in means the wire size has to exceed the amperage of the breaker.

It’ll be massive overkill. If it was me, unless I was planning on leaving room for expansion (ie also spec’ing the wire from the house appropriately based on potential future current load) I would return the panel and save some cash.

I did something similarly, but I upgraded my primary panel from 100A to 200A - and so I had a free salvaged 100A panel that I turned into a subpanel.

It came in handy for me because I was able to add HVAC into my outbuilding when COVID hit; I had the extra capacity (including wire gauge) to install a mini split and resistive heating.

If you’re only ever expecting to use the items listed in your post, you definitely won’t need the 100A panel though (and the requisite 100A wiring).

Note that most municipalities will require an electrical inspection whether you as a homeowner or a licensed electrician does the work. NEC (electrical code) in North America will also have a lot to say on the depth of buried cable etc between your house and shed, and how you mark it. Not following code and not getting an inspection is a recipe for excess fire and electrocution risk, and risks insurance not paying out / you may have liability in certain cases.