Uploading images in comments will be in the next update, along with a fix for the inbox count bug! We're also working on a Mark Read on Scroll option, a Modlog page and support for moderator actions (which have all been highly requested features).
Unfortunately, full markdown support is rather tricky to do. We've done some deeper research into how we might implement this since the last time it was suggested; it's a large undertaking that would require some significant time input, but it should be possible. It's not something we're scoping for the upcoming update, but it's something we hope to dedicate time to further down the line.
Mlem dev here! We're still working on it - a PR is merged every couple of days on our GitHub. It's true that development isn't as active as it used to be in the height of the Reddit migration, which unfortunately leads to new features being announced less frequently, but that's to be expected. We're working towards another release sometime in the coming weeks.
If you noticed any bugs with Mlem and you'd like to report them, you can do so here and we'll look into it š
Side note: We don't currently have any plans to drop Lemmy 0.18 support. Supporting 0.18 isn't a major technical challenge for us at the moment, so we don't really have a reason to drop support for it.