cardboard VS PlotSquared

Are you mixing up PaperMC with Fabric? PaperMC indeed is a fork of Spigot that is performance oriented. Fabric is a Modloader just like Forge. Fabric does not have plugin capabilities, since it is Mod based. There are Projects, that aim to add plugins to Fabric, like Cardboard, but these are still a work in progress. As you can see by their Progress Tracker.

I play strictly vanilla Minecraft, and use paper to host my servers, so this may be out of my territory, but I do remember Cauldron being popular for Forge mods and spigot plugins, so maybe give this a go?

The two pinned issues on their github show how much has yet to be implemented. Just as an example, here's the list of events and see how many are unchecked.

I believe it's a mixin conflict between Create and Cardboard: https://github.com/CardboardPowered/cardboard/issues/350

As for Spigot plugins on a Fabric server, Cardboard does work, though it’s still in beta and incomplete.

Use Cardboard: https://github.com/CardboardPowered/cardboard

Some more detail on how the server internals will work: The server is a based on Cardboard which allows both fabric mods and bukkit/spigot plugins to run at the same time, along with datapacks of course. The server has a wide variety of lag reduction and bugfix mods, as well as some rudimentary anticheats to prevent xraying and other blatant unfair advantage hacks

Cardboard exists but from my experience it can be buggy: https://cardboardpowered.org . Use at your own risk!

git clone https://github.com/IntellectualSites/PlotSquared --branch 6.11.0

plotsquared if you can compile it

That plugin is open source, you can build it using gradle

It costs 15 euros on spigot, but you can compile the plugin for free from their Github

You can actually download PlotSquared from their github for free: https://github.com/IntellectualSites/PlotSquared/releases

clone plugin repository from github (run git clone https://github.com/IntellectualSites/PlotSquared.git command)