It has more to do with general popularity of your ex group...
That's a huge factor but it can't carry you, if that's all you have it will be a state of constant decline over time. If all you have is the pre-existing fanbase of your previous group without any influx of new fans the solo run will have an expiry date. I think this has been seen time and time again. Sadly I think we are seeing some of this with the IZ*ONE solos, as much as I love all of them. Across the board their sales seem to be declining, although even the declined sales can still be considered very good and more than enough to keep the solo career going for a long time.
Of course if you are looking at short term success this might even be THE most important factor.
One interesting note is that the solos of group members seem to do much better while the group is still active, before disbandment. This makes perfect sense since you have a huge fanbase supporting you while a large portion of those fans might move on to stanning other groups after disbandment.
The real test is having a successful solo career for years, long after your previous group has disbanded. Very few succeed at this and it requires something special.