Seems like a good idea - use [WAL mode](https://sqlite.org/wal.html), and UUID based filename to provide obfuscation for the path and it should be a neat encapsulated installation.
You could also place the database outside the web root. As far as I know WordPress already supports putting `wp-config.php` one directory up from the main installation.
Time series would be more beneficial for a significant user base of WordPress which is using WP as a de facto company record described as a CMS-CRM.
And completely useless for the most significant user base of WordPress which is using WP for their small marketing website or blog.
Why would a time series database be better for a CMS-CRM? Or am I completely miss reading your comment?