Thanks for your question Vlad
I guess the most user-friendly way would be like how uBlock Origin implements it, with an addition collection of lists that you can activate. But if that can be called minimal, I'm not sure about.
The most minimal way would be to add a 'Add Custom Blocklist URL' button in the Privacy section where you can go to a custom section/window where you can add the URL of custom blocklists.
A list of potential URL's: https://easylist.to/pages/other-supplementary-filter-lists-and-easylist-variants.html
What would be useful is that a user can see which lists are already added to Orion by default, to not add lists with duplicates that might slow down Orion.
Hope this helps.
Best of luck with development,
Jan