I’ve actually gone with Isso which is Python-based and uses a simple flat file SQLite3 DB to store the comments. It was a complete arse to get configured and running, but it’s nice and lightweight and it allows you to import your existing Disqus comments. So it seemed the best option.
This isn’t necessarily a recommendation for Isso, by the way. I’ve only just got it running and am still at the tweaking stage: https://discuss.gohugo.io/t/how-to-do-proxying-during-development/5517
So my internal jury’s still out as to how good a comments system it actually is.