I will first finetune the speaker set up in the new flat and then get a UMIK1 to take some measurements with REW. Based on those...
I'm a 100% measurements/perceptual/science guy. 30+ years of correlating measurements and sound. My "view" of the data will be very different than yours just starting out, simply based on experience. That said, by far the best investment you could make, is first buying this (e)
book.
You will (hopefully) learn there is a significant difference between a pressure mic (UMIK et al) and 2 ears/brain. Interpreting what those measurements tell you...and don't, is key.
My advice is to use your ears! Start with careful setup/placement,
listen/measure/listen again, etc...before you buy a single speaker-room bandaid like "treatments", Dirac, etc, etc.
*If* you chose decent speakers, you should only need a touch of EQ below 400hz or so. Judiciously. Furnish your room, add decor, etc.
Most of the time that should be plenty enough "treatment", as you will find ears/science found in the book (the opposite of anecdote, belief, fashion, etc, etc.). It's only after that, or in extreme scenarios that further measures would be needed. YMMV
cheers,
AJ