Is my understanding incorrect in that IDE was woven into the the architecture via SCSI hooks? ISTR @trag or @Gorgonops making that point many moons ago.
If I said that the Macintosh LC630 Developer's Note says I was mistaken(*); the IDE support software is documented starting from page 67 and it shows the new 68k "ATA Manager" implemented *alongside* the SCSI manager underneath the device manager, not nested underneath it. It does essentially replicate (a subset of) the same API calls so IDE disks mostly "look like" SCSI devices to a greater extent than might be strictly necessary for a bare-bones block storage device, but that makes sense from the standpoint of thus requiring minimal changes for client software that might hit the lower-level APIs like disk partitioning software, formatters, etc.
(* Not going to own making that mistake until I see it, but, eh, maybe I made it? I do remember at one point when we were talking about PCI IDE controllers discussing how the Open Firmware BIOSes of some of *those*, particularly the RAID devices, do apparently look a little more like "SCSI Devices", or at least "Generic Disk-like thing" than native IDE to the Classic MacOS, but that's a different kettle of fish.)
For a really brain-dead example of how to write a Macintosh storage driver, see how the "Sony" driver for floppy drives is reused in abused in various contexts. Most "classic" Mac emulators don't emulate actual storage hardware, they gut the Sony driver (which has the most basic necessary set of read/write APIs feeding into it) and turn it into a passthrough to a "generic" interface to read and write blocks from disk images. It's my vague recollection that essentially this same mechanism is used for those "Rominator" Flash ROM disk hacks for various Macs, but don't quote me on that. If you don't care like your storage device "looking like" a normal disk drive then, yes, you can make the driver pretty darn simple.
(I've seen source code for Macintosh RAM disk drivers that are only a couple K long; if you hung some kind of interface to a USB flash drive into the memory map somewhere and were satisfied with bit-banging control and data bytes in and out of it the whole mess could be pretty darn simple. But of course something like this would *just* be a dedicated storage device; a full hot-swappable USB stack able to enumerate multiple devices and attach arbitrary devices is a much taller order.)