It really depends what your overall goal is, and how much time you want to put into the game. Legendaries are the "easy" part, if you have the daily time to do the grind.
1000 essence = one legendary for your spec of choice.
PvE Sources
- Daily Heroic: 30 per day x 7 days = 210 a week
- Daily Emissary: ~30 per day x 7 days = ~210 a week
- Mythic +15: 120 a week
- AtBT Raid (LFR or higher): ~300 a week
- PvE total: ~830 a week
PvP Sources
- Daily BG: 30 per day x 7 days = 210 a week
- Daily Brawl: 30 per day x 7 days = 210 a week
- 1500 honor in 2v2: 30 a week
- 1500 honor in 3v3: 30 a week
- 1500 in RBG: 30 a week
- PvP Total: 510 a week
Total possible weekly essence (when brawl is active) = ~1340
That equates to easily 1 token legendary every week with a "bonus" token legendary every 3 weeks + any natural drop legendary items, which tends to be around one every two weeks with solid play-time investment. I don't recommend wasting your time getting the 90 essence from 2v2, 3v3, and RBGs unless you really love PvP as it is not worth the time investment imo.
Now if you have a main that has all their legendaries, you can get lucky with tokens as well and speed this process up.
With that said you said Mythic progress is your goal. Are you looking to get into a guild? Did you reroll within your existing guild? At this stage you have 4 months until this expansion ends, and probably 3 months or less until the pre-patch happens. I for one would not recommend burning yourself out on content that is meaningless in a few short months, and would focus on playing for fun and being ready for BfA.
Any guild just starting mythic progress right now likely isn't serious enough to warrant min-maxing of perfect legendaries anyway. You could play belt and ring on every fight as fire and still do fine compared to your counterparts, although not optimal.
As a side note - your frost relics are under ilvl 915 in that link. You can buy level 915 relics from the vendor on the Vindicar with the currency that drops from rares. You should really consider at least doing that.
Here are your stat weights for single target fights from raidbots in the gear you logged out in as well:
https://www.raidbots.com/simbot/repo...9NoHfwHD4Zj1Xt