Would you consider dual-licensing this under a FOSS license like the AGPLv3? That's strong enough copyleft that I think you'll be safe from what you're trying to avoid with your current nonstandard license.
But you don't have to trust me, the AGPLv3 starts immediately.
I'll update the source headers in the following days.
Thanks!
The custom license is a pretty basic derivative of the FSL-1.1 with MPL-2.0 instead of Apache (for a weak copyleft), I think I might keep the custom license in place (I'll make the very minimal changes from the stock FSL-1.1 clear).
But adding an AGPLv3 license for those with ambiguity concerns is totally okay with me (it's ultimately more restrictive than the source available license in a lot of ways).
Thank you for the great suggestion.