LogFAQs > #962358550

Apparently the MD subreddit already figured out a way to maximize the gems from daily missions in this game.

https://www.reddit.com/r/masterduel/comments/sjgc2i/monthly_gem_income_from_dailies_and_ranked/
https://www.reddit.com/r/masterduel/comments/sj0171/minmax_daily_missions_deck/

Basically, you have to go 3 days without redeeming any daily missions so you eventually have all 9 possible daily missions in your mission list. You can complete them, just don't redeem them to claim the rewards. Then, you just complete and redeem the 3 specific daily challenges that give the biggest reward of 50 gems (done by winning 2 ranked duels and completing at least 3) every day while not redeeming any of the ones that give smaller rewards. That way, the 3 missions that give the biggest reward get constantly refreshed and you can get the max of 150 gems each day from dailies. The daily mission list seems to clear out at the end of each month, so make sure to redeem all 9 missions at once on the final day before that happens.