I think the better solution here is to offer 10-man Mythic again (back in the day, known as heroic).
Yes, there may be situations where 10 man might be easier than 20, and vice versa, but 20 will still be seen as the "real" Mythic by the RWF community anyway. 10 man mythic (known as 10man heroic at the time) used to be a thing before WoD. It was awesome and allowed us to do the hardest content in the game with a smaller group of people, which made it easier to assemble a team. If it's that big of a problem for RWF, keep 10-man mythic closed for 2 weeks.
Re-introducing 10-man mythic difficulty would fill a void that currently exists in WoW's raiding landscape.