i websearched for some time. the chipset is SC9832E, and looking for that chipset i found similar experiences to mine, where oem unlocking can be enabled but fastboot still fails.
there's some documentation of working around this for the qin 2 pro at https://en.typeblog.net/1c72/how-i-unlocked-xiaomi-qin-2-pro-and-installed-phh-gsi/
