Q) Do I really have to keep all the traces the same length, as critical as 3 mm?
A) Yes it is important that the I/O traces are routed such that they have identical lengths, within ~ 3 mm, to assure equivalent propagation delays. To assure reliable data transfers for all configurations it is important that the propagation delays for the clock trace and all data traces are identical.
Q) Will it help then to keep it 50 Ohm if I don't even know what is on the output of my SPI CLK pin (impedance-wise)?
A) Yes clock trace should have 50 ohm char impedance.
ok, well that confirms what is in your article, not like I really doubted it, I just wanted an explanation why, especially about the 50 Ohm impedance, since If my pin has different impedance than 50 Ohm, even if you tune it to 50 Ohm a 1000 times, there will be reflection... hence - weird. But nonetheless, fine. Any other suggestions about the layout?
No other suggestions about the lay out.