Honestly, you can ignore the Imperative books and just play core Mythras, by the original book. There are a few rules improvements in Imperative, but many just play with the still-solid core system in the original hardcover and are more than happy with the game.
Many see Imperative to a "starter set" for the system, and still go by the original book for the core experience. Until we get a revised core book under the ORC license, this is how it will go, and that is cool.
As gamers we tend to "want everything perfect" but the best we can do is good enough. Plus, I bet The Design Mechanism wants to sell through its current stock of OGL hardcovers before committing to an ORC release. Be patient, and support them as best we can as a community. Making YouTube videos and writing blogs helps!
The same goes for Classic Fantasy and Classic Fantasy Imperative, while not to dissuade people from getting the Imperative books, but if all you want to play with is the core book, you will be fine and have a great time. The Imperative books are streamlined and simplified, so the opposite opinion exists, though most of the spells and monsters you will need for classic fantasy are in the original book. Plus, it is far easier wrangling one book than two.
If you are an advanced player, know the system like the back of your hand, and want to incorporate the changes, that is fine. If you are just getting started with Mythras, just get the original books and live in those for a while and learn the game from a single source. You will have fun, and the system was as solid as it was back then as it is today.
Mythras has a surprisingly strong YouTube community, and rock-solid player support. This is a well-loved game with a hardcore player base. This is not a crowd that will jump ship for 5E or other games easily, the players are lifetime players and deeply committed to the game. The ACKS II game and this are in a battle for Mythic Bronze Age mind share, and Mythras' support is bedrock solid with its players and they won't cross over to a d20 system that easily.
If feels like Mythras and ACKS II have the market split, with Runequest trailing along in third. Not in sales, but in the interest and third-party support that I see. Mythras has been picking up some huge new releases these days, especially with Black Lodge Games and Cults of Zahak, an amazing release and showing an impressive maturing of the market.
All the cool stuff these days is outside of 5E.
As for 5E, forget it, none of the Bronze Age 5E settings have taken off, mainly because most D&D players don't want to give up their pseudo-Renaissance identities and Tiefling horns. Mainstream D&D has stopped being role-playing and has crossed over into a specific brand of cosplay LARP, and most players don't want to change genres, which kills alternative setting support and interest. This is actually a good thing, since Mythras does the genre far better than a video-game-like system.
If you are getting started, don't feel bad about just getting the original book and beginning there! All Imperative means is the system is in active development, and a new license and basic-support version was needed for 3rd-party support going forward. If I am out here writing adventures for the system and want to be able to sell those books going forward, forcing small publishers to use the poison-pilled OGL is a non-starter. It is ORC or nothing, and Imperative is the way to go for future support in expansions and adventures.
As for playing or getting started?
Start with the original hardcover and you will be fine.