To summarize:
\set -args..
EXECUTE prep
Is ok to do, but \SET command takes long when the args include a large JSON like the one you have. This is due to
https://jira.issues.couchbase.com/browse/MB-65181
You can download the version of the tool with fix in the upcoming release 7.6.6/7.2.7
Also, another tip to reduce the time taken is to set the history file to be /dev/null using the hist flag Couchbase Docs.
Hope this helps:)