It's up to you.
For $150 the flowvis may give us some more data. It may confirm your actual flow is less then the heater requires. Then you will need to dig into your plumbing.
Your problem is either external to your heater in your plumbing or internal to your heater. It takes someone hands on trying different things. I might disconnect various pieces of your flex hose and visually examine it on the inside. I might run your pump and see the amount of water going in or out of your filter or into the heater.
Maybe the heater needs to be disassembled and the internals examined.
If you are not capable of digging into your equipment then you need to find someone locally.