Luis said:
Perhaps than you can help me, because (besides the disappointment I have basically lost many connections between records and now I have a huge mess)
I was not using recID.... 

If you are using a dynamic choice then you are using record Id, that is where the problem lies when the table gets defragmented on import.

Now this problem only happens if the root table has lots of deletions. If moving forward your Categories table will be stable then you will not have this issue.

To use JSON, your dynamic choice formula could look something like:

let xCat := sort((select Categories where Active = 1).'Category Name');
for cat in range(0,count(xCat)) do
    {
        id: cat+1,
        caption: item(xCat,i)
    }
end

Now the dynamic choice is not linked to a specific record Id. So as long as the returned Categories records are the same then you will not have to worry about. But once you add new ones or delete others then that will screw up the dynamic field.

That is why I made the comment about using dynamic choice fields as only an UI element. Then put in the trigger after update code that updates a table or another field that stores the data you need.

Luis said:
From this perspective, the dynamic choice doesn’t bring me a lot of benefits as I can do the same with “relationships”

That is why I keep mentioning dynamic fields as a UI element. I'm guessing you are entering data directly in the tables. If you were to start using dashboards/pages to create data entry views then that is where dynamic choice fields come in handy.

My DB is a personal DB, but I have slowly started to use data entry views instead of direct. It seemed like a lot of work to do, but it saves me from having to jump around tables to do things. I can also implement error checking (cause you can never trust the end user).