howto prepare mac os ventura to use external disk for user data and backup with NAS located time machine

If your user home is on an external, you don't need to Migrate. Just create the same user as you had and set that folder as your home folder as you did, previously.

You need to keep that admin user on the internal when the external fails to mount in time to load your user at login.

You'll need to consult your HAS as to how to back up with Time Machine. You should just need to make an SMB share to use for Time Machine and find it in the Time Machine settings.

/System/Volumes/Data is where your data volume (Macintosh HD - Data) is mounted. You should not need to know that or access your data from that path. It is not a Snapshot. The startup volume is mounted from a Snapshot.

Well, I guess my question was, why did you migrate? Makes no sense.

But, it sounds like that's a better solution than you manually pointing your home to there.

Are you sure it isn't /System/Volumes/Data/Users? That's normal.

There is no longer a method to restore both the OS and the data. Apple’s instructions are for the standard setup that they support. Except for an Active Directory network home, you will not find any Apple instructions for moving your home folder to another location, so you are responsible for knowing how to restore your unique, unsupported configuration.

I don’t know what Migration Assistant would do or has done as it isn’t supported. Since you moved your home folder to an external drive, I expect that you know how to do it, again. To maintain your desired configuration, you would need to create a new user on the new or erased Mac using the same name as your user home folder name. Then, change the home folder path to the external’s location.

You should have no expectation that Migration Assistant would handle your configuration in any normal way.

Unfortunately using such a custom configuration is very risky, especially these days since macOS seems to be breaking such configurations (even when just putting the home user folder onto the external drive), much less using custom links to do so. Supposedly macOS 13.3.1 is supposed to fix some issues with some of these custom configurations on external drives. macOS these days does not like surprise configurations which deviate from defaults.

I'm not entirely clear on how you customized your setup which is one reason it is nearly impossible to advise you beyond what @Barney-15E has already suggested. I was going to suggest another possible option, but after thinking about it I don't think it will work. The best & easiest thing to do is to perform a clean install like @Barney-15E suggested. Manually set up the Mac again by creating your macOS user accounts and installing your third party apps. Then launch the TM app and pull your files into the new user account(s)...if possible as I'm not sure whether TM will allow this with all the new security settings macOS now has.

When you perform such a custom setup, you are ultimately responsible for figuring out how to fix things when something breaks. Apple only handles default setups and I think Apple barely even tests the default setups these days by all the bugs I've encountered with each release in the last few years. Like I said before, without being entirely clear on your custom setup (or its current condition), nor how TM may behave in such a situation, I am not sure how anyone will be able to provide any better advice than you have already been given. Things like this usually require being able to see the system and the errors. It is nearly impossible to convey all the necessary information to us here. You may even have to go through multiple attempts to fix things anyway depending on whether you are trying to fix the current setup or start fresh.

I just thought of a reason TM may be having an issue with your custom configuration....perhaps TM is attempting to transfer an APFS snapshot of the home user folders instead of just slowly transferring files like it used to do. This is just a guess though.

I wish we could really offer more. If you figure out how to fix things, then update your thread so it may help others in a similar situation as yours.

Good luck.

Thanks for the follow up. I can understand those two custom setups. While both of the proposed setups should theoretically work fine, be prepared for macOS to possibly break things with an update. It has happened multiple times recently.

And thanks for providing the information on how to get TM to restore to such a custom setup. I just learned something new.

Good job figuring it out and thanks for sharing.