I have no desire to do the math but 24 runics/week for 300 gems over 52 weeks buying the 6 non-Sunday diamond packs seems far better than any runic drop rate you’re going to get from those equivalent gem/VIP chests. And when you have 1-4 of most every legendary (many already ascended) getting a random one that likely isn’t one you need doesn’t excite me. Event keys used strategically are far better at getting specific legendaries in my opinion.
There’s a mistake in your post. You assumed that daily Gem Bounty gives 2 Gem Keys and therefore its cost is reduced by 2*9 = 18 gems resulting in 32 gems. So one Mythic would cost 32 * (4000/24) = 5333 gems.
But there is no Gem Keys in regular Gem Bounties. One Mythic is
50 * (4000/24) = 8333 gems, not 5333.
It used to give the equivalent of 2 color specific gem keys but then the summoning stones got nerfed.
Soul forge hit when i was about level 900. I’ve crafted Infernus, Famine and Pharos-Ra so far, and i’m about 1200 diamonds away from crafting Elemaugrim. Before Infernus, i had no mythic at all. Now i’m level 1231 and own a little bit less then half of the mythics. It’d not have happened if i hadn’t bought almost all sunday $5 packs, aswell as gem bountys, and i also buy 2-3 random $5 packs every week. My RNG is horrible, so i’ll happily pay up just to be certain that i eventually will be able to catch up on mythics about once every 5 weeks.
I’m doing VIP chests until I have 10 of them (Mythics). After that I’ll only do VIPs during exclusive periods, 9000-13000 gem stash is enough for that. Any surplus above that will be spent on Daily Bounties.