I think I found the answer after (a lot!) more searching here:
https://stackoverflow.com/a/64047575/3663863
Although the better option might be to move away from packages.config or find better nuget libraries that don't have so many old dependencies as suggested in comments, I suspect I'm like a lot of people - stuck with what I've got and just needing to fix it.
The original blog post that answer links to is worth a read, as well as the one it got the answer from found here: