NetBoot Server - How to store the netboot images on a shared disk?

I'm a long time user of apple's NetBoot system. Although this system is totally dead on new machines, I still use it from time to time when I'm working on old systems. My home server is a 2012 Mac mini formerly running 10.11. On that OS, I ran everything normally without any problem. For other reasons, I've had to upgrade that mini. It is now running 10.15. So I set up a 10.11 virtual machine and installed the Server.app on that. This all works just fine with one catch: I do not want to store all 200+ GB of netboot images inside the virtual machine's disk image. I want to store those images natively on the file system of the host machine, the 10.15 machine. According to the NetBoot docs, you are supposed to be able to choose to store NetBoot Images, NetBoot User Data, or both, on any of your physical hard drives OR NFS shares. So ultimately what i'm trying to do is a lot simpler than it sounds. I'm trying to run the netboot server on the 10.11 virtual machine, using images stored on the 10.15 machine. The fact that one is virtual doesn't really matter. What I'm going for should work the same even if these were two separate physical Mac minis, one with 10.11 and one with 10.15. I thought all I needed to do was share a folder on 10.15 over NFS, connect to it from 10.11, and it would show up and be ready to use. But alas, that did not work. The more I searched, the more it seems there are actually many ways to share and mount an NFS share, and I can't find any documents specifying exactly HOW the NetBoot service or the Server.app wants the share setup. Like everything Apple server related, the docs are SLIM and so is the number of users who use it. To be clear on my progress, I am able to share a folder on the 10.15 machine over NFS, and I can connect to it on the 10.11 machine. But it never shows up in Server.app as a possible storage location for the NetBoot (NetInstall) Service. This is Server 5.2 on 10.11. Any tips would be greatly appreciated. If I can't get it going this way, I'm going to have to move all of the images INSIDE the virtual machine which will be a real big mess I would rather avoid! Thanks!