As others said, spin down the drives when they’re not in use. Make sure power saving is enabled on the drives and tune them to spin down after some appropriate amount of time. (hdparm lets you customize it on Linux)
Consider also sleeping the NAS when not in use. You can try using Wake-on-LAN to remotely wake it up when you need to use it. Saves on electricity and heat! You could also sleep it on a schedule, in case you need to be online for backups to run at particular times.
Licensing representation matters
It doesn’t, because they’re the copyright owners. Think of their software as dual licensed: They run it themselves under a proprietary license, under which they reserve all rights. That has nothing to do with the AGPL version that they license to you. The AGPL doesn’t take away the rights they have as copyright owners, nor does it preclude dual licensing.
(Are you a bot? Your reply is written like ChatGPT, and it has that self-defeating logic that ChatGPT has sometimes… eg. you wrote that you disagree with me, but then parroted the exact thing that I said.)
This is flat out wrong. If you’re the copyright owner, you’re not licensing the code to yourself. The AGPL is the license under which they’re making the open source version available to YOU. The version they run themselves is proprietary.
Look into DeltaChat
I’m late to the party here, but did you consider just paying for Mattermost? If it meets your needs, and your organization has 250 people, the cost for licensing is going to be relatively small compared to your IT budget (right?). They have “contact us” pricing, which means you can negotiate it.