WBFillPause
(The plugin is tested from 1.13 - 1.19.3)

WBFillPause is a small plugin which simply pause WorldBorder's fill command when a player joins the server, then resume it when nobody is online.

The plugin is also compatible with Dynmap, the plugin will automatically detect if Dynmap is installed and use Dynmap rendering if enabled in the config.yml.

The fill command will not resume immediately when the player disconnect, but after 15 seconds.
This is made to prevent to re-launch the process if the player had a short connection loss.

How to install
Drop the jar file into your plugin directory, then restart your server.

Permissions
wbfp.bypass: Players will not pause the fill command. (default: op)

Credits
Special credits go out to arboriginal for creating the original WBFillPause plugin, found here.