We’ve released an update to SpinupWP that will enable SFTP/SSH access on a per-site basis and provide security isolation between your sites by running each site as a separate system user. Upgrading is a multi-step process that involves first running some upgrade routines on the server and running different upgrade routines for each site. This allows you to upgrade one site at a time and test each site after each upgrade. But each site and server upgrade should only take a minute or two.
You will not be able to deploy new sites to a server until you have upgraded the server. Nor will you be able to manage existing sites via SpinupWP until they’ve been upgraded.