You do not want to do this, I assure you. The TOAST / DE-TOAST overhead alone would wreck performance, plus OP said it was update-heavy; Postgres can’t update a JSON object on disk, it just rewrites the entire blob.
Postgres does this for _all_ row updates; it must, for transaction isolation to work.
A schema of `party1 JSONB, party2 JSONB, ...`, or even `parties JSONB` would likely be far more maintainable and sensible than the "table-as-spreadsheet" approach.