How to get clientId in google analytics with high accuracy?

Question

JavaScript

All documented and not very methods seem to have been tested.
I wrote a bunch of options for all sorts of different scripts that worked very badly (data loss> 20%), as a result, I came up with options that, it seems, should not depend on my curvature (?):
I tried it through custom dimension, according to this article:

gtag('config', 'UA-12345-1', {
  'custom_map': {
    'dimension1': 'clientId'
  }
});

Data loss has dropped to 15 percent, which is still unacceptable.
Then I went through gtm, something like this for the
first couple of days of data loss did not exceed 5% and everything seemed to be fine, but over the last week the losses returned to their standard level.
Additionally, I will clarify:
Custom dimension on the Google Analytics side has the "User" scope;
By "data loss" I mean users who got into Google Analytics reports (for example, the user made a transaction), but to whom the clientId was not assigned (i.e. when generating a report with clientId, these users simply do not get into the report).
The volume of traffic on the experimental sites is not too large, and does not exceed 500 thousand sessions at the resource level for the date range used, i.e. I don’t fall into the documented limitations of Google analytics.
Honestly, I’m starting to think that the data is lost on the side of Google.
Perhaps someone faced a similar problem?

Answer 1

I managed to set up the 3rd method from here