Yay, made it to mythic! That only took about 45 hours of insane grinding, lol.
The most important thing I found last few days was adapting to an ever changing meta, which can be fun but also very frustrating... all of a sudden you are on a winstreak, and a bit later you try again and nothing seems to work anymore.
The deck that pushed me the last bit on the ladder is a Selenya tokens deck (a mythic player on reddit recommended it), which I adapted a bit to suit the aggro-heavy meta I met in diamond. It seems very powerful to me and a good match for many of the decks you meet out there. If anyone wants to give it a swing:
3 Emmara, Soul of the Accord (GRN) 168
4 Hunted Witness (GRN) 15
3 Trostani Discordant (GRN) 208
4 Flower // Flourish (GRN) 226
4 Saproling Migration (DAR) 178
4 March of the Multitudes (GRN) 188
2 Pride of Conquerors (RIX) 17
3 Settle the Wreckage (XLN) 34
4 Legion's Landing (XLN) 22
4 History of Benalia (DAR) 21
4 Conclave Tribunal (GRN) 6
8 Plains (XLN) 261
4 Forest (XLN) 277
4 Sunpetal Grove (XLN) 257
4 Temple Garden (GRN) 258
1 Memorial to Glory (DAR) 244
I also used a WW deck to grind the lower diamond ranks, it's this stuff (thanks to a streamer on Twitch called Wincon):
4 Benalish Marshal (DAR) 6
4 History of Benalia (DAR) 21
3 Pride of Conquerors (RIX) 17
4 Skymarcher Aspirant (RIX) 21
4 Snubhorn Sentry (RIX) 23
3 Squire's Devotion (RIX) 25
3 Legion's Landing (XLN) 22
4 Adanto Vanguard (XLN) 1
20 Plains (RIX) 192
4 Rustwing Falcon (M19) 36
4 Hunted Witness (GRN) 15
3 Venerated Loxodon (GRN) 30
Good luck to anyone out there still trying to get higher up!