I believe the only way to firmware up an Xbox controller is on an Xbox or through the Xbox Accessories app on Windows. I've been using my work laptop to update my Xbox controllers.
As an alternative, some of the 8bitdo controllers are in LVFS, and I've heard the PlayStation Controller Firmware Updater works in WINE for DualShock and DualSense controllers.
You must have had a real sweetheart deal on VMware then. Proxmox is cheaper than VMware even under the old pricing. You also don't have to buy the "Standard" subscription. There are cheaper ones.