A good way to earn gold is to pace yourself. Set daily goals on farming. Example: Farm 100k worth of crates per day. It will not only keep you farming, but it will also keep you from over-exhausting yourself. You could also try merching (buy low/sell high).

Mythic items are said to be good for 1 extra season. The level 36 mythic sets would last during the lvl 36 season (last season) and the lvl 41 season (this season). So they most likely will be outdated by the release of the next expansion.