Hi Alex_Sk,
Greetings, Stage upgrades where mostly for legacy devices when major changes were introduced like ( jloader etc) this is not applicable for your platform, you can go directly to the code you desire, furthermore the QFX's don't have the configuration validation option so you might want to test this in the lab at least with one spare device to make sure that your configuration will be compliant with the new code( this is the only way you can validate your configuration) please check below KB: https://kb.juniper.net/InfoCenter/index?page=content&id=TSB17508&cat=&actp=LIST
Regarding NSSU, my friend, I would not recommend this also NSSU won't work from any 14 code skipping 2 major releases only 14 to 14 or 14 to 15, this will just take more time and you will still have high chances of running into some issues please check: https://www.juniper.net/documentation/en_US/release-independent/junos/topics/reference/general/nssu-qfx5100vcf.pdf
Divided approach fro the upgrade; I believe that this is the best way to go, if you are more concerned about the downtime and not on all of the steps and the time this would take you to accomplish the upgrade, please note that this will reduce your network capabilities/operation to 50 % since half of your devices will be down at some point ( pushing the code to the file system snd rebooting) at least for 15 to 30 min.
Make sure that each group of 3 has a RE ( spine device) and that should be it! good luck!!
If this solves your problem, please mark this post as "Accepted Solution" so we can help others too \:)/
Regards,
Lil Dexx
JNCIE-ENT#863, 3X JNCIP-[SP-ENT-DC], 4X JNCIA [cloud-DevOps-Junos-Design], Champions Ingenius, SSYB