How can I stop exhaust fan vent condensation problems?
It's a near certainty to be the insulated flex duct. For the sake of discussion, insulation will help quite a bit, but some drip back is inevitable due to the configuration. Condensation will happen. Bathroom exhaust has a very high dew point. As soon as it hits a surface colder than dew point, condensation happens.

With insulation, condensation will occur much higher up, but it will happen. Then it runs back down the duct to drip out of the fan into the room. The only solution is to not vent straight up. The duct should turn to horizontal as soon as possible after leaving the fan. The remaining duct should slope slightly away from the fan to eliminate drip back. If possible, the duct can be exhausted out a nearby sidewall. If the roof exhaust is the only option, the duct could be configured to have a type of drip loop where water can collect instead of running out of the fan. Hopefully any water collected would evaporate before much can accumulate.

You might consider placing a very small drip hole at the lowest point, and let condensation drip out of the duct into a more suitable container from which it can evaporate.