" No fiberglass or plastic "

" and acrylic (not sure of durability) "

You realize acrylic is essentially plastic..........

Acrylic is fine. Cast iron is fine. Composite "stone", which is acrylic and dust, is fine. All just need to be not the cheapest thing on the planet quality.

I do realize that Acrylic is in same family as fiberglass and plastic. The current tub is Fiberglass and is 35 years old and it has held up well. My concern with cast iron is that it is heavy. This hall bath is tiny. The door is only 27" wide. Space will be difficult to maneuver to get tub in.

Seems like you've ruled out ----everything.