It's possible to handle these kinds of fields (at most one value) with separate tables and foreign keys. It has advantages around 'proving correctness'. But without extra tooling it has significant usability, and perhaps performance disadvantages. I wonder if you can get around some of this using virtual tables.

I come at this with little experience with SQL, and having worked a bit on ampersand[1], a tool where you declare the structure of your data, and some invariants, and the tool will create a schema for your database, and some automatic checking to ensure your invariants are upheld.

[1] https://ampersandtarski.gitbook.io/documentation/