How to determine preload resistance needed for an ATX power supply to power up?
The apparent difference is that the CD drive loads both 5V and 12V supplies, while your experiment only has the fan on 12V and variable resistor on 5V.
It may simply require more loading on the 12V rail.
And thus it just takes a lot of experimenting what kind of dummy resistor loads should be placed on 12V, 5V and 3.3V output rails and in which proportion.
Not all ATX supplies will work well as a benchtop supply. Since it is an ATX power supply, power must be drawn from ATX supplies according to ratings specified in the ATX specifications. For example changes in load current must not happen instantaneously but with a certain limit how fast it changes, so that the supply regulation feedback loop can react to it without too large overshoots or undershoots in output voltage. Basically the best option would be to build a buck/boost converter which takes in power from ATX supply according to ATX specification and provides an abusable outputs for lab experiments.
Therefore, a dedicated lab power supply works better than expecting an ATX supply to work under non-standard loading.