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considering it's a booting/running system already, I leave the existing EFI alone and don't reference it at all int he install?

Yes, better to leave it alone

What about grub? Should I "upgrade" it at all? It is still the same grub I install 16.04 many years ago. lol

When you install a new OS, grub will likely get overwritten with both old and new OS options available

if I create a `/home` in it's own space, `/boot` in it's own space, and then put `/` in the final partition space, will that put all the usual linux directories on the `/` partition?

If you understand the concept, it is straightforward. "/" is the only directory that needs to be mounted, everything else is optional. That is, with "/" mounted, you can, if needed, mount any other directory under "/" into separate partitions. Typically "/boot" and "/home" are mounted into separate partitions for easier maintenance.

Going with #3, is this a fair structure?

248MB EFI (leave alone)

200GB OLD INSTALL (leave alone)

512MB /boot

If you plan to keep your system updated, I would increase /boot to 1 GB. You will likely run out of space after 4 kernel updates.

80GB /

200GB /home

leaves 30GB free

I'm not using any swap, I have 64GB RAM installed and don't need it

Keep in mind that, if you want to hibernate the system, or for the hibernate option to work. You will need Swap. If your regular RAM usage stays under 16GB, you could simply just create a swap partition for 16GB.

Hope this helps.