What I would like to know is what flash mode that would fit this setup best. Should i simply set it to "M" and adjust it on each picture depending on the results, or should i use TTL? Or perhaps none of the above?
There's not a single correct answer here other than do what works for you. If TTL gives you results that you like, great! That's what it's there for, right? Otherwise, manual mode is fine -- adjust parameters like the flash power, flash zoom, and distance to get the lighting you want. The point of manual mode is to let you set the flash the way you want when TTL isn't cutting it.