Time Machine not thinning old backups
I gave up trying to fix this directly, but after upgrading to macOS 11, and using the same SSD freshly formatted with APFS - as now supported in macOS 11 - TimeMachine is back to working as it should, with hourly snapshots being cleared out after 24hrs.
I can't say if the upgrade cleared out something corrupted, or if macOS 11 can now deal with the issue I was having, but it did fix things.