I think maybe you are barking up the wrong tree trying to convert substack into a citable academic platform. The primary cornerstone of academic literature is peer review, and that pretty much defines what gets a recognizable citation in something like Scopus or Web of Science or Google Scholar (I worked at Google for a long time but I retired). The mere presence of a DOI isn’t enough to make something citable, because it provides little more than a separate namespace for stable URLs. It has also been somewhat muddied by the fact that datacite issues DOIs for code and data, and arxiv issues multiple DOIs for different versions of the same article.
There is a significant amount of scientific discourse that takes place on other platforms and doesn’t contribute to citation counts. Preprint servers like arxiv, medarxiv, biorxiv, eprint.iac, and many others fulfill another role of disseminating academic literature without peer review, but they tend to be more formal. Preprints grew in response to paywalls by academic publishers, which limited the distribution of academic literature.
I think of substack as being more informal than that, but that doesn’t mean we should dismiss it. There has been a huge amount of scientific/academic discourse on social media, and it now occupies an important role. That goes back to things like blogger, twitter, wordpress, etc. Academics now often write about the importance of social media for scientific discourse. Even before the web there were email lists like LISTSERV and usenet groups. As we all know, twitter was dramatically upended recently, and as a result a lot of academic communities just dried up there (much to the dismay of journalists). Some migrated to activitypub/mastodon or bluesky or whatever, but it fragmented the space. This may represent an opportunity for you to grow your site, but keep in mind that many sites are angling for it.
I regularly interact with a lot of these blogging or micro-blogging platforms, but I will personally never click on anything on medium or substack, because they are behind annoying login walls. I also won’t invest any effort in posting on such platforms because it limits my audience. This now includes twitter, which recently instituted a login wall. The equivalent of academic citations in the informal web world is the hyperlink, and SEO is now well understood as the mechanism to curate this. Unfortunately if you want to get links, then the stuff has to be readable. Otherwise few people will link to it and few will drive traffic to you. I think wordpress is taking a much better path by integrating with activitypub/fediverse. It doesn’t require a login to read stuff there, and using the fediverse will greatly extend their reach. They are following the lesson learned from preprint servers, which allow people to read things with minimal effort and have no login required. login is reserved for authoring, discussion, or other features.
I don’t wish to dissuade you from trying to gain more academic reputation and capture scientific discussion. More to the contrary - I think it plays an important role. I just think you should recognize what you aspire to and what could potentially hold you back. In my mind login for reading is a complete non-starter.