Correct. If you've already committed to being a non free player, then you should use all your available mythril for stamina refreshes (or inventory) and gems for relic pulls. Of course, if you want more stamina refills than you have mythril available, then you've got no choice. But just looking at the conversion rates, since 100 gems gets you closer to a relic pull than 1 mythril, use the mythril for something other than relic pulls.
If you're staying free, then you have to balance the mythril between everything, obviously .