Not true. Being portable from server to server (even across different file and operating systems) is one of DokuWiki's main goals. All you may have to do is clear the cache. See https://www.dokuwiki.org/faq:servermove
It’s somewhat difficult to update and requires frequent security updates
DokuWiki releases come out like every 1 to 2 years. Security hotfixes are rare but always easy to apply (usually you just replace one or two files). Upgrading is more or less a couple of clicks when using the upgrade plugin: https://www.dokuwiki.org/plugin:upgrade
It’s design, and all the available templates I’ve found look pretty old
I guess that's up to personal preference. Personally I'm still happy with the default template but people seem to like the bootstrap3 and argon templates
It’s native format is not MD
True. There are plugins that add MD support, but I agree, native DokuWiki syntax is much better supported. If you're familiar with Markdown, it won't be hard to pick up DokuWiki syntax, but yeah it's different. DokuWiki is about as old as Markdown, when it was invented markdown was just another unknown formatting language...