If I were doing that job, I would buy the lumber from a lumber yard and get 9/4(2&1/4" thick) rough sawn lumber that is half the total width needed. I would plane to the 1&7/8" thickness, joint two edges, cut to nominal length, edge glue two pieces to make each tread, cut/trim to fit, finish, and install.

You might be able to get wood in 20" widths. If not, edge gluing is actually stronger than the wood. My planer is only 13" wide(as are many home shop types), so to plane a 20" wide piece would need to be done by the lumber yard.

Most full service lumber yards can machine what ever wood they sell.

Thank you! We are in SF and finding reliable companies, due to current inflow of major money makes it a bit challenging. Do you know of any companies that can pre-make the treads, since they can easily be removed, thinking that can be done fairly fast. Thank you again for any suggestions.

Thank you !

Plywood? really, I'm surprised as plywood has a lot of deflection vs say a hardwood like red oak. I am going to assume it is engineered for the load but are they springy?

If they are thick enough, even plywood can be pretty solid.