Joplin is great and I use it extensively. But Obsidian has many functions that make it a better choice for a Zettelkasten. These include the "[[…]]" notation (known as "wikilinks") for the rapid insertion (and creation) of links, auto-completion of those links, automatic generation of backlinks, and the ability to create a dashboard by dividing the Obsidian window into panels.
You said, "I was tempted to download Obsidian to get the graph UI they offer but I couldn't make myself move off Free Software like Joplin." You are mistaken, Obsidian is free and always will be. Like Joplin, it has an enthusiastic community that develops powerful plugins.
If you are interested in having Obsidian host your notes on their server you would have to pay for that. But if you use Dropbox or similar hosting platform, you don't need a paided hosting solution. If you wish to turn your massively linked set of (Zettelkasten) notes into a website so that other people can access your Zettelkasten online, then Obsidian provides "Obsidian Publish" to fill that need. This function is of course external to Obsidian, therefore you can choose to pay their subscription fee for internet publishing. But for all comparable functions to Joplin, Obsidian is free.
Joplin runs on mobile devices, which is an advantage it has compared to Obsidian. Obsidian's roadmap contains mobile versions, but that apparently will not be implemented in the near future.
All in all both Joplin and Obsidian are great solutions for effective (and free) note taking. They differ in the specific problems they aim to solve. However, each can be "bent" to solve almost any note taking problem.
As an aside, it is important to me to heavily link Joplin notes. I hope Joplin will adapt the wikilinks approach to make note linking more rapid and flexible, This will move Joplin closer to becoming a Zettelkasten solution. The Bear notes app used to use a similar method to Joplin to link notes. A year ago they added wikilinks, which greatly improved its ability to handle Zettelkasten notes. I hope Joplin does the same.