Upgrading to separate components and speakers is going to give a much better experience. But sometimes simplicity wins the day. I would recommend upgrading to a traditional setup based on performance alone. But if it’s a soundbar form factor that’s more important, I believe Klipsch, yamaha, and nakamichi(others too) have SB’s that utilize real surround speakers(not fake ones in the SB) and might be a way to split the difference. Those also might allow the addition of a higher quality subwoofer.
Imo, it’s time to move on. The JBL tech was right. There’s no way to add any speakers to that system.
Out of curiosity though. What were you interested in adding?