This is a valid point. Was it a screenshot during fluctuation? Can it hold 600 RPM when warm, +- 5-ish RPM?
What is your alternator output voltage at idle? Use a DMM, not the dash gauge.
I would want to make sure the valves are seating. Using a vacuum gauge, you can connect to the evap port on the TB, or the port on the intake resonator, I would use the TB.
If the needle is steady, meaning a very slow, gentle swing, almost imperceivable, when the TB is controlling the idle is good. If the needle bounces back and forth rapidly, even a little bit, that would indicate an intake valve not fully seating.
Take an index card, or envelope, hold against the exhaust flow of the tailpipe, the exhaust should keep it pushed away. If it slaps back to the tailpipe, then you might have an exhaust valve fully seating.
I would still try another known good coil one more time, only since you have done other things since then.
Make absolutely sure the coil connection is good, use some electronics cleaner but MAKE SURE the connector is DRY before re-connecting.
My go-to in a situation like this will always be a can of BG44K, not any other treatment, but 44K specifically. Then I would re-evaluate after a tankful. It's pricey, but no snake oil.
Still not ruling out another plug/coil if all else fails. After that, then more in-depth digging will be needed.