Freescout probably fits the bill and is 100% free - it's one of those where they give the base platform away and monetize plugins.
On the flipside, the plugins are lifetime licenses are very cheap relative to the monthly charge levied by competing products.
https://freescout.net/
Outline and Grist
Can anyone who's successfully running Grist within their Outline instance respond here or PM me? I have some questions about how it's supposed to work.