Ah, I get how you're rolling up category spending from that line-time table, so having a one-to-one transaction-to-designation record is expedient. Am unsure how I'd otherwise structure that, but a thought is that you could set the spending categorie...
I'd be interested in checking this out. I developed this household budget Google Sheet which maybe someday I'll find the time to rebuild on Airtable. You're welcome to steal from it, but it takes a very different approach of using transaction tables ...
RE: "I know I'll need AT LEAST one item for every transaction." ...why not just structure your main table with both a line item amount and split line records, then configure a formula field to use the former when there are no splits or the sum of ...