Here is the order in which eighth edition codices and supplements were released including the second release for Space Marines and Chaos Space Marines:
Space Marines
Chaos Space Marines
Grey Knights
Death Guard
Adeptus Mechanicus
Astra Militarum
Craftworld Eldar
Tyranids
Blood Angels
Dark Angels
Adeptus Custodes
Chaos Daemons
Thousand Sons
Necrons
Tau
Drukhari
Deathwatch
Harlequins
Imperial Knights
Space Wolves
Orks
Genestealer Cults
Chaos Space Marines (2nd release)
Chaos Knights
Space Marines (2nd release)
Adepta Sororitas
Ultramarines (supplement)
White Scars (supplement)
Iron Hands (supplement)
Raven Guard (supplement)
Salamanders (supplement)
Imperial Fists (supplement)
It’s interesting to scan the list noting when the stronger codices dropped, some of which were factions along with their accompanying supplements. Of course the overall of strength of certain factions waxed and waned over the course of eighth edition due to the effects of things such as errata, FAQs and re-adjusted point costs for units via Chapter Approved.
We know that Space Marines and Necrons will get their codices first for ninth edition which indicates that the following new releases will not follow the original cycle, plus there has been some rumors regarding new factions possibly coming. Also typically Space Marines is the first faction to receive a new codex for the start of each new edition.
It appears the rate of release for codices will be much slower for ninth edition due to the pandemic and this has an effect on the meta as opposed to the rapid fire release at the beginning of eighth edition. While eighth edition had its indices, ninth edition has the Psychic Awakening expansions for the various factions. Ideally all factions can start a new edition now at roughly the same strength, but of course this is really not the case.
Will GW release codices this edition in an order that will provide better external balance between factions? This is what most want but being honest we know this has never been the case. Which factions need the most help - for example it appears a good number of the xenos factions need the most help, such as Orks and Tyranids.