Going with a single network is never good. Even if your fillrate hits 99%, your cpm will go down hard as it will repeatedly serve the same ads.
So either cherry pick them yourself or look for a mediation platform.
IronSource is hard to use if you start having issues. We had sudden crash and ANR rates above 5% with IronSource (they never answered us) and switched to Ogury (which we met at gamecom so we have direct contact to their staff, instead of support mails that never get answered).
Ogury is also one of the few that have a working Admob mediation going (IronSource never worked for us), which is ultra important in our experience (13$+ CPM in many regions).
They also built us a custom ANE (took not even a day), stripping out some play libs they honestly shouldn't use bundled in the first place.
Ogury is currently looking for testers for their new GDPR implementation, so that might still be worth a look for people concerned with this.