I hear org-mode is a good alternative. In the mean time, consider storing a permanent .html version upon publication of markdown
Markdown isn't the be-all and end-all of markup formats, and there are valid reasons for using formats with strict specifications (e.g., AsciiDoc or ReStructuredText for certain kinds of technical writing). But the chances of Markdown documents suddenly being woefully unreadable in 20 years are roughly in line with the chances of plain text Unicode documents being unreadable in 20 years.
Notably, GitHubbers and StackOverflow representatives joined to define CommonMark, but GitHub and StackOverflow do not have remotely similar fenced code block conventions.
Also, CommonMark currently says that they have not yet defined a version 1.0 of the spec.
I certainly applaud the effort, but CommonMark did not unify all the Markdowns.
If your documents are available on the open web, you can visit them on the Wayback Machine to make sure they're cached as HTML. This is in some ways more reliable, and some ways less, than trying to keep track of your own HTML transcodes over decades. It's generally a more reliable way of making sure they're available after you disappear.
The major point of using markdown, or why I use it, is that it doesn't require a working implementation to be understood. I can open a markdown file in ed and still understand it. From what I've seen of RestructuredText and Org-mode, if i'm not using the implementation, it becomes a mess.
(I also always recommend to put anything that isn't widely implemented in Markdown in HTML instead since that will be reasonably parsable for the next couple decades.)
What are the threats to your content availability — hosting provider censors you, hosting provider dies, your cloud servers die, your on-premise servers die or their storage is corrupted, your workstation dies or its storage is corrupted, you miss a bill, you lose your ability to pay bills at all?
How likely are these? What happens? How much do you care? How could you recover and re-publish your data? How quickly do you need to recover, if at all? How much (time, money) is it worth investing now, to reduce the risk of future data loss or the duration of an outage?
What does "your own platform" mean? Without relying on things like Tor, I don't believe it's possible to publish anything without relying on some large corporation who can pull your content with a day's notice. If not Medium, it'll be AWS or Google Cloud or your domain registrar. Anyone who cares about an open web should be, at minimum, calling out those who take down content on short notice without reason.
I agree we should call out sites for censorship, but there's a strong case for people taking more control over their publishing.
Nearlyfreespeech and Gandi comes to mind here, I'm sure there are others who try to set as clear stance about their TOS. In general for web hosting though you should research that they have good connections, don't over-provision and have competent staff.
It's a huge pain, especially now that most platforms are closed and don't provide automation (APIs, RSS), but it's really the most flexible solution.
It also makes syndicating to new providers a lot less painful since you'll have some standard raw form (e.g. Markdown) which you can create a manual or automated pipeline for.
Considering Liberapay is using Mastodon, I expected them to already know what they're getting themselves into. Especially with Medium's thickening walls.