Isn’t the definition of “observant” closely related to intermarriage?
If you perform observances based on the “norms/rules of Judaism as laid out by your branch of Judaism”, or based on your community norms, or based on family tradition, it’s easy to rationalize deviating from those norms/rules if a good reason emerges.
But if you perform observances because they’re in the Torah that G-d gave to Moses, deviating is not an option. And the Torah specifically prohibits intermarriage.