This machine is externally accessible.
The value stored in your TrueNAS is the actual data and that is why you should keep it safe. Should this data now be in that Ubuntu system, it is now that one that should not be accessible from outside anymore because it is now that one that has the crown jewel.
I keep my private computer backups and files directly on truenas.
Then you should have two completely different and independent backups plans and solutions instead of trying to merge them.
I have a second virtual disk connected
I hope this is poor translation by Google.... Never use any kind of virtual disk with TrueNAS. TrueNAS must always have direct and complete physical control of all its drives.
So now, I recommend you to stop trying to build anything. You need to detail a complete plan.
How many different category of data do you have : private, public, configs, softwares, ...
Where are these data : Windows, Mac, Linux, laptop, in the LAN, from Internet, ....
What volume does that represent ?
How often do they change ?
How these data are used ?
Are they used concurrently by many users at a time ?
...
Once your needs are defined, a proper solution will be possible.
Example of very different solutions would be :
--Stop having any data client side and start managing everything server side (Ex: Nextcloud)
--Have dedicated backup agents installed on the end devices and have these agents syncing to TrueNAS / Ubuntu
--Have each user do his own business in his own dedicated and isolated environment
But what you are trying to do (have all users interacting in a kind of half-shared infrastructure using custom-built sync and access tools) is surely not the best way to do it,