There is no option for drives in the Optiplex 380 other than "enable" or "disable" and AHCI is not mentioned in the service manual anywhere.
Running gfxutil gives me this:
00:00.0 8086:2e30 /PCI0@0/IMEI@0 = PciRoot(0x0)/Pci(0x0,0x0)
00:01.0 8086:2e31 /PCI0@0/PCI1@1 = PciRoot(0x0)/Pci(0x1,0x0)
00:1b.0 8086:27d8 /PCI0@0/HDEF@1B = PciRoot(0x0)/Pci(0x1B,0x0)
00:1c.0 8086:27d0 /PCI0@0/PCI2@1C = PciRoot(0x0)/Pci(0x1C,0x0)
01:00.0 10de:0fff /PCI0@0/PCI1@1/GFX0@0 = PciRoot(0x0)/Pci(0x1,0x0)/Pci(0x0,0x0)
00:1d.0 8086:27c8 /PCI0@0/USB0@1D = PciRoot(0x0)/Pci(0x1D,0x0)
00:1d.1 8086:27c9 /PCI0@0/USB1@1D,1 = PciRoot(0x0)/Pci(0x1D,0x1)
00:1d.2 8086:27ca /PCI0@0/USB2@1D,2 = PciRoot(0x0)/Pci(0x1D,0x2)
00:1d.3 8086:27cb /PCI0@0/USB3@1D,3 = PciRoot(0x0)/Pci(0x1D,0x3)
00:1d.7 8086:27cc /PCI0@0/pci8086,27cc@1D,7 = PciRoot(0x0)/Pci(0x1D,0x7)
00:1e.0 8086:244e /PCI0@0/PCI4@1E = PciRoot(0x0)/Pci(0x1E,0x0)
01:00.1 10de:0e1b /PCI0@0/PCI1@1/HDAU@0,1 = PciRoot(0x0)/Pci(0x1,0x0)/Pci(0x0,0x1)
02:00.0 14e4:1692 /PCI0@0/PCI2@1C/ethernet@0 = PciRoot(0x0)/Pci(0x1C,0x0)/Pci(0x0,0x0)
00:1f.3 8086:27da /PCI0@0/pci8086,27da@1F,3 = PciRoot(0x0)/Pci(0x1F,0x3)
00:1f.0 8086:27b8 /PCI0@0/ISA@1F = PciRoot(0x0)/Pci(0x1F,0x0)
00:1f.1 8086:27df /PCI0@0/pci8086,27df@1F,1 = PciRoot(0x0)/Pci(0x1F,0x1)
00:1f.2 8086:27c0 /PCI0@0/pci8086,27c0@1F,2 = PciRoot(0x0)/Pci(0x1F,0x2)
I'll try out the latest Lilu / AppleALC when I get a spare evening.
As for Big Sur, to be honest it doesn't really do anything I particularly want. Only reason I'm looking at upgrading this past High Sierra is because support is getting towards EOL. Actually, Catalina is a deal breaker because it won't run a bunch of 20-year old Windows games (that work fine on High Sierra) that my kids like playing. Seriously, that's one of several stupid moves by Apple who seem to be going out of their way to reduce customer share, convince them that switching from Windows is bad, and that jumping ship to Linux is an increasingly interesting option.