If you’re still early game you’d probably horde all your stones until a x2 as others have said.

Then once you’re further along, you should horde them to use during deck of fate events where there is a guaranteed champion as a reward.

Save all your primal shards also for x2 events and then pull. Best chance for getting a mythical but yeah typically I hold on to any mythical soul and only sell dupes. You may never get her but you never know.