I considered it, but when I started the project I was using a Supabase DB for the backend since their free-tier is quite nice, and after the switch to and from BigQuery, PostgreSQL was the easier migration. I also thought Postgres might be more suited to the backfill/ingestion job due to the frequent row-level reads and writes. That said, I know DuckDB can use Postgres in the backend, and I might consider that if the current model starts to struggle.
I have also seen some benchmarks that suggest the gap between DuckDB and Postgres isn't always so substantial: https://jsonbench.com/#eyJzeXN0ZW0iOnsiQ2xpY2tIb3VzZSI6dHJ1Z...