The "installer not checking you had 35 GB free then leaving your Mac unusable" issue only applied to upgrades from earlier major versions (e.g. Catalina) to Big Sur. It was fixed with the Big Sur 11.2.1 full installer, and probably the 11.3b2 full installer, since that was released a week after 11.2.1.
Updates to a later minor version of Big Sur (via Software Update) use a different mechanism and were not known to have this problem.
Minor Big Sur updates may still require a fair chunk of disk space, because they keep an APFS snapshot of the previous version during the update. I haven't seen guidelines on how much free space is required but I can imagine something in the order of 25 GB may be needed in the worst case, based on the size of the system volume of my real Macs running Big Sur.
I haven't created a Big Sur guest yet, but I have kept copies of the earlier installers so I can do some experiments when I have time.
