READONLY is for accessing the underlying TS db. To publish you must use a User account.
Correct I am using my user account publishing, it works fine with other data sources like Oracle in 2018.2.3
I am now looking into if the data access is disabled by default in 2018.2.3 - I just output the configure file and I don't see a line in there.
Maybe another thing that even if you port the data into 2018.2.3 from 10.5 it takes some of the data but not all the configuration?
I had to run this command
tsm data-access repository-access enable --repository-username readonly --repository-password
Now it works.