OcBinaryData VS OSX-ALXEthernet

HfsPlus.efi is missing, get that from this link and then put it in your Drivers folder: https://github.com/acidanthera/OcBinaryData/blob/master/Drivers/HfsPlus.efi

Fixed loading macOS with legacy boot without Apple Secure Boot: there is a new PE (Portable Executable) loader that broke legacy boot, blocking the loading of legacy drivers as HfsPlusLegacy and preventing OpenCore from booting on legacy systems when Apple Secure Boot is disabled. With this fix, legacy boot can work with or without Apple Secure Boot. This issue was detected after version 0.8.8 was released. While you are on 0.8.8, you must download the new versions of the drivers HfsPlusLegacy and ExFatDxeLegacy (the one/s that are necessary), available in Acidanthera. This is not required if you are on OpenCore 0.8.9.

Update Resources as well: https://github.com/acidanthera/OcBinaryData

Check if you have the latest resources Folder (included in OC Binary Data)

Copied from the latest OcBinaryData

I'm able to download and open up the zip as instructed, but as far as dropping the "ExFatDxeLegacy.efi" file into the right location on my ESP (which I understand to be my boot drive, where OCLP was originally installed), I'm at a total loss.

Have you tried downloading the version from https://github.com/acidanthera/OcBinaryData ?

I'm also looking into a replacement for AttansicL1Ethernet, but that seems to be all there is for your old ethernet controller. For what it's worth, the official source for that is here: https://code.google.com/archive/p/iats/downloads Maybe ALXEthernet would work better?