Pekak, i went through a list of reasons why rabbonim usually tell people in that situation to remain married. But can you say that it’s an ideal for a person to have as their life partner someone who will not help them, and often hurt their progress? It’s just that the potential fallout is cumulatively worse; the BT might have regrets, might drop the whole thing, the frei spouse might get custody (especially if it’s the mother who’s not frum).
Why on earth would it be ideal to have a non frum spouse??