I think foam vs. homasote is one of those issues where everyone has their own opinion and there is no universal best way. Certainly, supply chain issues may impact your decision. In addition to the big box stores, homasote may also be available at smaller, local building supply stores in your area.
Also, if you're going to use a topper, you may want to re-consider using a hardwood veneer plywood as a base, since it will not need to be super smooth and you could save a little money by going to a cheaper plywood.
