… I am using a 2TB NAS on network at present and it is a pain sometimes (slow to load, etc.).…
I have a twin drive 28TB NAS that I use for Time Machine for the whole network. I would never load files I actually use on it. As you already know: Life is too short.
I prefer the APFS volume over the external drive, it is faster in my case. I have created both. Creating the volume was easy, to create the bootable drive with OS Catalina install I used a third party software, still easy to do.
You're right, adding a volume is a lot faster but I would lose the ability to do near-instant TM restore via APFS Snapshots on my System drive (doesn't work with two MacOS on the same System*). With some of the apps and testing I do, losing Snapshots is not an option for me. Since Snapshots isn't an issue on my external, two volumes on one drive is fine.
*On the occasions I have to restore via Snapshots, I shut down then turn the external off before I boot ⌘ r into the Repair Partition. Can't do that with a partitioned or multi-volume drive. Again, not an issue for most users.