Now I can't help but wonder if there should be a `waln` mode where the WAL files would round robin instead of alternate between just two potentially allowing for much more intense write cadence.
Other databases do do similar to what you suggest, though obviously the trade-offs will differ because of other different internals and product priorities, so it would have been thought about. For instance MS SQL Server has multiple “virtual logs” in its log files, for at least some overlapping reasons.