As you have noticed, automations only trigger for records that meet the condition after the automation has been turned on. Here are some workarounds
If there are only a handful of records, you can manually “test” the automation for each record, picking a new trigger record each time until there are none left. This is feasible only if there are very limited records, and if the automation changes the record so it no longer meets the condition.
Adjust the trigger to temporarily include some other condition that you can manually trigger. For example, create a temporary checkbox add add a condition like “or temporary checkbox is selected”. Then after turning on the automation, select the checkbox for the existing records.
One of your conditions is when a particular field is empty. You could temporarily paste in a placeholder value for a few seconds so the records do not meet the conditions, and then delete that placeholder value so that the records do meet the conditions and thus trigger the automation.