Thank you very much, Pastrychef -- as always, you are very helpful. My PCIID for the USB 3.0 Bus is the same as yours. I assume I should insert that in the .dsl file instead of a12f, i.e. "8086_a2af"?
I have installed the .aml file, without that change, and deleted the 'change 15 port limit ..." entry in KextsToPatch. I do have the APIC entry, and also the USB 10.13.4+ by PMHeart -- I assume I still need that. As it stands, all 26 USB ports still show in IORegistryExplorer attached, but as far as I've checked the ports I use are working. though somehow some entries have changed, e.g. the port used by Bluetooth. Maybe that because of the wrong PCIID?
I'll redo the guide Identifying Ports process. I'm still on 10.13.5, because when I tried 10.13.6 I lost USB 3, even with the 10.13.6+ patch from PMHeart and FredWest. I'm hoping the SSDT will fix that, when I get the SSDT right.