i always recommend using only PCI-based or firewire-based audio solutions for OS9; I always found USB audio in OS9 to be troublesome. for sure, the lack of USB 2.0 would have something to do with it. I second the recommendation for MOTU and RME, with an emphasis on MOTU because they are very old players in the Mac game and their drivers are rock solid across any Mac OS generation. I personally dont really remember RME being a major player, at least in the Mac space, until the G5 era, but their reputation is also great.
Having said that, MOTU still supports their legacy devices with their drivers so even the oldest MOTU 828 interface would still work in Mojave. That's part of why I love MOTU stuff. The 828 MK2 is probably your best bet in that regard; the original lacked MIDI. Doing a sold item search on ebay, it looks like if you're willing to be patient you can get one for around a hundred bucks, and its an interface that can grow with you thanks to the digital ADAT and SPDIF IO. MOTU also going to be cheaper on the used market because there's more of them running around; they're much more mainstream (and less high-end) than RME is.
this thread on oldschooldaw also talks specific interfaces with OS9 support, it may be useful. heck you could get one of the Trashcan, err Tascam (i keed) and have a mixer control surface!
http://www.oldschooldaw.com/forums/index.php?topic=9.0