In a 4-layer board design, the recommended layer stack order from top to bottom is: signal, ground, power and signal.
Bypass capacitors should be placed in close proximity to the VDD and VIO pins.
The length of the START and STOP traces from the DUT to the stopwatch/MCU should be matched to prevent uneven signal delays. Also, avoid unnecessary via-holes on these traces and keep the routing as short/direct as possible to minimize parasitic capacitance on the PCB.
Match the length (or resistance) of the traces leading to the RTD sensors. PCB series resistance will be added in series to the RTD sensors.
Route the SPI signal traces close together. Place a series resistor at the source of SDO (close to the DUT) and series resistors at the sources of SDI, SCLK and CSB (close to the master MCU).