In a roundabout way you’ve explained where I was going wrong - I needed to add those site params to a new file/folder structure (config/_default/params.toml) rather than adding them to the hugo.toml in the root (which I thought was setting the site params)
Yes, outputting the results of the collection was trying to debug, to see why the toc widget wasn’t displaying despite the params being set (in hugo.toml)
What did I want? To have the toc display
What did I get? The toc not displaying
But placing the params in that separate file (that I didn’t know needed to exist) resolved it.