Question 1 What Color is the LED light? Question 2 is it flashing at all? If its not flashing and if its white than its more than likely your HDMI Encoder chip is damaged, your fuse that is near the HDMI port is blown or the HDMI port is bad.
it sounds like you know your way around a system pretty well. It’s odd that you would be having those issues even though followed proper procedures (Assuming everything was done correctly). I know this might sound odd. Try to reapply thermal paste. Come applicare la Pasta Termica As much you probably don’t want to go through that again. I’ve repair many PS4’s but never had any do that Thermal Compound i use is Arctic MX-4 (2019 Edition) If you figure it out im really interested on what the reason is
i would take out the components and see whats loose. If there is nothing loose then check to see if there is anything broken. If you don’t know if you feel comfortable taking it apart you can always go to your nearest service center. I doubt it’s the HDMI Encoder or else it wouldn’t show up at all Slim: Smontaggio della PlayStation 4 Slim Regular Sony PlayStation 4 Repair Pro PlayStation 4 Pro Repair