SPI bus connections - to loop or not to loop?
Loop back? NO. If the line is long enough to need termination (longer than risetime*c/10 or so), then drive it strongly enough to end terminate it properly, and match the line and termination reasonably well. If the line is short enough not to need termination, then it won't need the 'extra' conductivity that you seem to be looking for with the looped back connection.

A series resistor? NO. That style of source-end termination only works for a single point receiver at the end of the line. At earlier points on the line, you get the worst possible waveform for your receivers, which is a step to half voltage, followed by a dwell, followed by another step to full voltage.