You’re right. Multisite is required for auto-generated sub-websites, however, multisite isn’t required to demo pages for wp-plugins. Two points here:
- Not all wp-plugins are compatible with WP multisite; and
- You can demo wp-plugins using a single WP site.
You may be trying to bite off more than you can chew here. If WP and Caddy are new to you, you might find it more manageable starting with a WP single site - serving WP PHP files using a Caddy webserver and Caddy reverse proxy. There’s a lot of good information in this forum you can tap into e.g. Example: WordPress and the Caddy forum moderators are extremely helpful. Just be aware, they’re Caddy experts and will do what they can to help you make your applications web accessible and secure, but they are not necessarily versed in any application forum members are trying to integrate into Caddy.
Once you have WP single site working, you’ll be in a much more informed position to consider transitioning to multiple WP single sites (uncoupled from each other) or WP multisite (tightly coupled sites).
I started with multisite last year, but then switched to multiple single sites. FWIW, here’s a blog post with my reasons for switching WordPress: To Multisite or not?. Personally, there’s only one situation I would consider multisite and that’s for building a multilingual WordPress site.