I like how the SDP creates a few more subdirectories that can be symlinks / directory junctions to map them to different volumes, while they are all accessible directly from the "instance directory". I notice that some of the configuration necessary to do this is inconsistent, e.g. in order to separate the database from the versioned files (absolutely recommended), SDP sets an environment variable P4ROOT and a configurable server.depot.root.
Add a configuration parameter "P4INSTANCE". If this is set, change all the defaults for P4ROOT, P4LOG, etc. to be relative to this, and follow the SDP convention (while allowing to override if specific values are given). No further configuration would be needed for any directories.
When introducing this on an existing installation, since all the original configuration is already there, nothing would change - but a new server would have a simpler configuration. I'm not sure how to deal with an SDP update - but since everything is so nicely layed out to begin with following decades of experience about how to configure a Helix server, maybe that's not an issue.