The Scythe Ninja 5 might be a quieter cpu cooler.
Test results with the fans running on maximum (12V) and 130W cpu-simulator from hardware.info:
Dark Rock Pro 4 - 49,2 °C - 41,8 dB(A)
Ninja 5 - 53,1 °C - 30,4 dB(A)
Simple and cheap, similar to Jonsbo C2
Under 10 liter
Top IO: power button, 2x usb-c and 3.5 mm
140 mm case fan
Magnetic dust filter
Bottom to top airflow
Easy access to both sides of motherboard (maybe with a slide mechanism similar to DeskMini A300?)
The smallest mATX I know of is the (Cooltek) Jonsbo C2 with volume of 12.10 liter.
I had some mATX motherboards (or was it the GPU) that didn't fit and put those in the bigger 18.82 liter Coolcube Maxi...