A simple approach that may work is to create an Automation Rule that intercepts the rates created by our Automatic Currency Rates feature.

Normally these rates are automatically updated (ie: daily) but a rule such as this can modify the updated rate to force it to be fixed:

You would then see that each daily rate ends up being stored at the fixed rate:

Although each transaction in Odoo uses the "latest" rate in the system, it will always be the same until you archive or edit the Automation Rule:

Be sure to set the domain properly on your rule to just act on the EUR rates.