I've been using SuperBetter for 6.5 months, all of that time without an Ally.

That was a conscious choice - from my past experiences with accountability buddies I knew that it's not for me. After the initial "honeymoon period" it just adds more distractions, stress & mess to my life. My self-care must be private.

But that's an individual thing I guess.

As for finding an Ally,

1. You can try posting your request on reddit, in /r/GetMotivatedBuddies subreddit, for example. And if your challenges are niche (specific to particular health issues, profession, lifestyle), find a relevant subreddit (google for it, use tools like redditlist, ask in /r/findareddit) and post there as well.

You can also try posting in /r/SuperBetter/ or write a comment to the latest post there (find it by sorting posts by "new", called "Accountability Partner"). It seems dead though.
You mentioned you don't have any social media. reddit is the safest, most nourishing social media site, I'd say. And it's anonymous (unless you register a username that has your real name in it).

Oh, and you can make reddit look minimalist by using its old design. In preferences go to the very bottom and uncheck "Use new Reddit as my default experience" check box.

2. Jane McGonigal (inventor of SuperBetter) has a blog post about finding an ally, with links to niche sites/communities. Google for "Where to find allies online" "Jane McGonigal" (I can't post a direct link to the blog post, because this feedback platform autodeletes all comments with links in them). That post is back from 2015 though, so some of the linked places might me dead or have better alternatives now.