Generating 50k rows of unix time entries auto-incremented 5 mintes each
Plan A:  (Drawback: you might still have gaps.)


Find all epoch values from the 4 tables using UNION DISTINCT.
LEFT JOIN from that table to the other 4 tables.


Plan B:  (Drawback: I don't say how to do step 1.)


Build a TEMPORARY table with every 5-min time for the next N months.
LEFT JOIN ...


Plan C:  (Drawback: Step 1.)


Upgrade to MariaDB.
Use a "sequence" table to 'virtually' generate enough integers.
Use some simple arithmetic to convert the integers to epochs.
LEFT JOIN ...