my solution to this problem is a OS X Leopard 10.5 virtual machine running in VMWare Fusion. I have a "shared folder" in the VM which points to a folder in my home directory (this machine im on is 10.14 Mojave)
the 10.5 VM is set up with AFP sharing enabled, (10.5 was the last OSX release to support OS9 filesharing. it was also the first major OS X release to be Intel-native, and hence can be put in a VM.)
On my MDD, I just went to AppleShare in Chooser. I did have to type in the IP of as it did not show up in the server list, but as soon as I hit connect I was presented with the classic OS9 server login screen.
Save the credentials when you log in.
Once you're logged into the 10.5 server and the volume is on the desktop, create an alias and put it in your startup items folder in system folder.
Every time you start up the OS9 mac, it will (provided the 10.5 VM is running on your regular machine) automatically mount that folder on your desktop.
(you'll also know immediately if you forgot to turn on your 10.5 VM because the OS9 machine will freeze up for 2-3 minutes while it is failing to connect to the 10.5 VM)
If you dont have any concerns about security, you could even make the shared folder in the 10.5 VM your desktop, or wherever your project folder is. Files would be immediately usable after transfer.
the VM could even be hosted on a Windows machine if that is your main machine preference.
with an SSD in my OS9 MDD G4, and transfers across a gigabit network... it is ridiculously fast. the best read/write speeds on an OS9 SSD setup are in the same ballpark as what gigabit ethernet provides, so really this solution is about as good as it can get for quick file transfers.