I have got most of my mythics from gem and glory keys. Once in awhile I get one from guild chests. However Drathas is right it does get better once your get the guardians all to mythic x4
The reason it gets “better” is you are only using 50% of the chest openings for a non-GG troops prior to obtaining 4x Mythics across all 6. (The other 50% are solely pulled from the 6 GG troops).
I’ve had all Guardians at mythic for at least 6-7 months. I’ve even bought seals and spent 4000 at a time and got all blues and 3-4 purples. I guess I’m just cursed.