Isso (https://isso-comments.de) is a commenting system,
which you can add in an
<iframe> below your blog posts,
after having installed Isso on a server of yours (there's no SaaS).
Both Isso and Talkyard are open-source. Isso is more light-weight, so you can use a cheaper server if you self-host Isso. It's one of the oldest commenting systems, and is still being maintained: https://github.com/posativ/isso
Talkyard has, unlike Isso, some features useful for larger discussions. And can function also as a discussion forum, should you want that, and there's a hosted option (SaaS) if you don't want to manage your own server.
Price and Hosting
If on-premise. SaaS always auto upgrades.Yes