THere are various scan tools that can read the BMW specific error/status codes, so, with some knowledge, you do not need to be a BMW dealer's shop to at least be able to get an idea what may need fixing. Things like brakes are pretty straight forward as are the suspension bits. If you have the REx, an oil change isn't rocket science, nor is the muffler. There are some strange bits in the fuel system, and they may not be competent to deal with charging or battery issues, but that's not a given, either. Consider, though, that in a dealership, they may only have had a few of their techs trained on the i3, so as a system, not every tech can deal with them. When it might come to body work, especially when it might involve the CFRP chassis, probably best to not deal with other than the factory trained people (and I'm not sure they can get all of the frame parts, either).
2011 535i x-drive GT, 2014 i3 BEV
Soon (hopefully!) A 2021 X5 45e will replace the above