The missing test payloads are where I would start the migration fixtures, particularly around conditional truthiness. An absent field, null, an empty string, zero, and false can select different branches after moving between template dialects, even when the normal preview renders identically.

A template with an optional amount and a separate boolean flag would expose those distinctions without needing a large email collection. I would compare both the rendered content and which conditional sections appear. That gives the conversion a behavioral contract, rather than treating successful HTML generation as evidence that the original template's meaning survived.