First, you have them backwards. You are a photographer. You learn Lightroom first for 90% of the work. As an instructor, you learn Lightroom Classic (forget the others). YouTube is great but KelbyOne may be your better choice as they approach it as you are photographer and as you develop how to use Big Brother PS when needed. Do not fall into the trap that PS Camera Raw is identical to LRC. Yes, they run on the same engine. No, they are not the same. Send me a PM and I will send you a booklet I wrote for a refresher course last summer.

Rich - thought I'd throw my 'tuppence' into your ring, even from t'other side of the Atlantic !

I've used Nikon DSLRs, but effectively 'went mirrorless' in 2012.

1. In terms of future proofing, go mirrorless rather than the D780.
2.If your prime motivation is to be able to capture movement then look past the Z6ii to the Z8 - best AF (along with Z9) at present on a Nikon mirrorless. Be prepared to explore the means of paying for it if you need to - save a little longer - what can you sell that you don't really need/use?
3. While F lenses will work on Z cameras, you would need to spend on an FTZ which you may not use for long. Take a look at your current lenses, explore which might be best 'in the interim', but start now to learn about Z lenses with Z cameras and plan for future redundancies with 'F' and purchases with 'Z'.
4. Bit of good thought and planning now may help prevent any damaging effect upon your wallet !

You may already be familiar with Thom Hogan, but you can't really beat his advice about 'all things Z' if you immerse yourself in his website ....... https://www.zsystemuser.com/

Get yourself in the right frame of mind about 'mirrorless' and have lots of fun as you move into new territory whilst using your long-time Nikon knowledge and familiarity - wish ya' luck from afar !

Not really a clear understanding of how it works. Folders are based on the directory structure on your computer when you copy a file into a folder the operating system looks for free space to save that file to and creates an entry in the FAT, (file allocation table), that is a pointer to the location where the file is stored. When you import into LR if you use the “add” option it creates an entry in the LR catalog with a pointer to that location. If you “copy” or “move” when you import it puts the file in whatever folder you specify and adds an entry to the catalog. You’re right that that file remains untouched. All of the edits are also saved to the catalog and when you pull up an image LR applies those edits so you see the edited image. If you opened that image in another photo viewer you wouldn’t see the edits. Collections are NOT virtual copies. They are simply pointers to the same image file in the folder. Collections simply allow different virtual groupings of images. Whether I open an image from the folder or collection it is opening the exact same image with the exact same edits. If you edit an image in a collection then those same edits will apply if you open the image from a folder. Now one thing you can do in LR is create a virtual copy. All the virtual copy does is add an entry to the catalog that points at the same image file but it also creates a separate copy of the edits for the new entry so you can make different editing choices. It’s a great feature because it allows you to try different editing choices without affecting your previous edits.