Can my belt drive frame be converted to a derailleur setup?
There is no good way. One could imagine ways of contriving a low-end claw-type derailleur on. Don't do that.
Nexus hubs are usually reliable. In some places in the world, there are lot running around without issue. That you've had chronic issues suggests either damage, defect, or a setup issue.
Some number of Nexus and Alfine bikes have made it into the world with mismatched cassette joints and shifters from the factory. It is a maddening problem until you've figured it out.
There are steps between what you have now and Rohloff, if it came to replacing the hub. I recommend taking it to a shop that really knows internal hubs. They are robust but have their quirks and unique procedures and things to check.