In general, it sounds like you’re trying to send a GA Session ID as a user property and also register it as a user-level dimension within GA4. GA has its own way of managing sessions, so injecting your own session ID might cause some discrepancies or confusion.

If you treat the session ID both as a user property and dimension, you wont necessarily mess up your data collection, but it may add complexity in understanding the information and you may retrieve some non-intuitive outcomes.

However, sending this session ID as a user property to GA4 could be useful for cross-referencing with data collected outside of GA, it won’t interact with GA’s already established measures of users and sessions.

In conclusion, while there doesn’t seem to be a data collection issue, you’ll want to make sure you fully understand how GA handles data in relation to users/sessions to ensure the most accurate and valuable analysis.