Could carved stone doorways have been meant as portals?

It depends on the specific site. Some Egyptian tombs had symbolic false doors, which were connected with the deceased and the presentation of offerings. That gives us evidence for a spiritual meaning, but not for an actual astral portal. A carved frame by itself can’t tell us that much without its setting, inscriptions, and related objects.

A ritual threshold is a reasonable way to think about some of these structures. A doorway can mark a transition in a religious or symbolic sense even when nobody was expected to walk through it. The leap from that to a place where consciousness literally left the physical world is much bigger, and the claim needs evidence specific to each site.

I’d separate two ideas that often get bundled together: a doorway may have had sacred meaning, and it may have served as an astral portal. The first can be supported by archaeological context or written records. The second is a modern interpretation unless there’s clear evidence for it. Similar-looking carvings in different cultures also don’t automatically mean they shared the same purpose.