Pool multiple drives into a single network share in macOS 10.14 or later
Solution 1:
Oddly enough macOs (thru Catalina) offers doing a JBOD RAID array (what you are describing) as a part of Disk Utility.
- Open Disk Utility
- Select RAID assistant... from the file menu
- Select Concatenated (JBOD) and click next.
Now there may be some limitations to making a disk pool with Disk Utility that I am not aware of as I have never actually done this myself but I would think it is, at least, worth investigating to see if it works for your setup and needs.