Thanks. That is excellent. Just to double check, does formatting the NAS drive to exfat allows the longest possible filenames and path length? Files I created using linux and unix systems 20 years ago had very long name and sever levels of directories.
Good that you double check before you are going to walk the wrong path.
You are confusing a NAS with an external USB drive. You can format an external USB drive to exFat, but
it is not advisable to do so with a NAS since a NAS runs an operating system (mostly Linux). A NAS is a file server visible in your network and determines its own disk format. Other computers in your network (your Mac, PC, tablets etc.) don't care what format the drive(s) in the NAS are, because they see it as another computer they can communicate with.
I advise you to stick with an external USB drive or even a flash drive. It seems you are turning a simple file move/copy operation into something as complicated as a rocket launch.
I assume you don't have a NAS (because of your question). A NAS is a whole different beast (basically a computer acting as a file server) and quite more expensive than a simple USB drive. At this point in time: completely overkill for your purpose.
Edit:
If you use an external USB drive and simply copy the files you want to keep from the Mac to a directory (name it 'backup') in the root of that external drive the long file/path names don't play a role anymore. Why do you want to maintain the long path/filenames? Isn't the most important thing that you can find those files if you need them?
Why don't you simply try it instead of seeing so many bumps on the road? Simply copy the files you want to copy to an external drive and be done with it. Your computer is not going to explode or anything. If some files won't copy for whatever reason, simply deal with it as it comes.
Also one thing I don't understand. You must already have backups from those files you want to keep I assume? Because keeping important files for that long only on an old Mac without any form of backup is asking for trouble. What would you do if that Mac fails?
If you already have backups how do you access those backups now? If they are in the cloud you already can access them from any computer you have. Or if you already made backups on an external drive you can access them already from any computer you have as well (if formatted in exfat).