To your point about psychological safety, that is actually reason to have an initial phase of idea generation occur individually before meeting in a group. Even a fully "positive-only" group likely inhibits divergent thinking to some degree.

Regarding groups, research challenges your claim that avoiding criticism during brainstorming "expands the boundaries of creativity", and that criticism is only useful for "winnowing down [already proposed] options." There are multiple studies showing that criticism during discussion _produces_ (not just selects) better ideas.

This is a good academic review of literature covering these topics https://pubsonline.informs.org/doi/full/10.1287/orsc.2020.14...