Just an update after calling my local Subaru and Toyota dealers.
Subaru - currently they do not have OEM 18's wheels for the winter. They only sell 20 winter tires. As expected those cost almost 2x as much as the equivalent 18s with no advantage.
Toyota - Looked up the spec, it looks like the Solterra has the exact same diameter, width, offset, centre bore as the 2021+ Sienna (I just took delivery for one of those).
Issue is that Toyota does NOT make Steel or Alloy 18 for the Sienna. They only have reasonably priced 17s steel or winter.
I think I might just have to get a set of 17 wheels and tires for the Sienna and use the Sienna's 18 wheels for the Solterra.