[PCB design] Layout for SPI bus due to ringing

Hi to all!
I always used SPI devices in low speed mode, now I need to push SPI clock at maximum device speed.
I'm using ADXL355 on evaluation board, directly connected to PIC microcontroller through 0.1" pitch connector.
On my first board I was using 0.3mm track width, 0.3mm track spacing and 0.3mm separation from ground plane.
Bus length was 18mm and I can see ringing but I can read at 6MHz clock speed.

On second design I decreased track width and separation to 0.18mm, increasing bus length to 35mm.
Due to misunderstanding there are no series resistor on SPI lines and rinnging encreased a lot, so I need to work at lower speed.

Can someone suggest me layout/solutions to avoid ringing on 0.18mm track width using 1.6mm PCB thickness?
Thanks.

Current layout: