What are some "professional" arduino alternatives?
If you're looking for something easy to graduate to from the arduinos, consider using an arduino without their IDE.  It's then pretty much the same as any other ATmega dev board. If you go to the Atmel website (now part of the microchip one) you can download Atmel Studio for free. This is a fully featured IDE based on Visual Studio. You can get a plugin for it which let it work with arduino files... but if you want to move towards more professional use, avoid it, and write complete C (or C++) files. For the ATmegas, with their very limited RAM, I'd stick with functional rather than object oriented programming.  I'd also avoid the using the heap/malloc/free unless it's really necessary.

From there, you can look at development boards, aka dev boards. That's the technical name for a microcontroller with the necessary supporting hardware on a PCB so you can try it out. They are available for many microcontrollers, though often only one per family of similar parts. They are more expensive than arduinos, partly because they are not made in such large numbers. Unlike Ardiuinos, they are also usually laid out by the manufacturer exactly how they intend with the chip, so they can be a good place to look for hints when laying out your own first PCB.