You can still run Mythic on your own with Farm or Selected mode, while using Heroic one.
The real problem is that Mythic groups are not straight forward, usually using group buffs, mesh or 3-seat passenger mounts glitches or other environment issues to skip this or that pack or entire bosses, so the entire sense of AFK mode is not possible.
The wipe-rate at mythic random groups is way higher than in random LFD, and 100% AFK mode is really not supposed in general in mythic dungeons, even with very high level of character gear.