I’ve been frugal with purchasing extra packs for the arcanes for the past… I’d say almost a year now. I’m still getting more Arcanes total than I am spending without resorting to explore at all (except a few events that needed it, which were for stones I already had 50+ more than what I needed). I coasted across the finish line from deficits of 60+ arcanes at some point, and the only time I ever overspent was on Arcane Blood, and only a little. We may be getting troops at an accelerated pace again soon, but who knows how many will come outside of glory packs thus increasing the amount needed without giving you more. Keep in mind that random arcanes also come from keys, especially VIP keys, so mythic hunts tend to give a glut.
I’d stop at the requisite copies of Sister of Shadows at mythic +4, if you haven’t gotten it already from hunting Magnus. If you have something you really want to trait, get enough for that. Or get one extra “set” of 16 Arcane Dark, and decide what to trait later. Spending 300 per stone for pretty much only the stone is just not efficient in the long run, so I’d only do this if you need to overcome a massive short term bottleneck. If you run into scarcity on both glory and arcanes, it is on average faster to farm a given amount of arcane than a given multiple of 300 glory. If you are going above what you need for the troop just for the arcane, you’ll get far more out of your stash if you spend on weeks where you can get 2 stones for 400.
As for 100k being enough “extra” to trait what you are missing if you spend all out on each one through the next several months, it depends on how many more you needed, how much glory you get per week, and how much you need to spend to cover just what enters the game on a given week. When I checked how many total Arcane stones were needed about a month ago, it was 4178. Assuming you need roughly the same amount of other stones as you need Dark aside from single color (so ~66 arcanes on 14 color combos, not counting Skull) and you earn about 10k glory per week (high kingdom stars, hourly collector) and that a third or less of the packs offered are Epics. This would be 2400 cost to trait the troop, 400 for 2, 66 extra arcanes, so minus 5.6k glory on these weeks, and others need 600-900 to cover the troop and 300 for one arcane times 66, so minus ~10.6-10.9k glory on the rest. In this (very) rough estimate you’d run into scarcity in about 11 weeks and fall short, but that is also making some pretty big assumptions like only having an average of one pack per week except for commons, how much glory you are earning which varies wildly from person to person, and how many arcanes you need which I doubt is anywhere close to accurate. You’d have to tailor that depending on what you have and what you earn. Keep in mind if you have to farm glory at any point to keep up, it was probably faster just farming the arcanes themselves, but if you hate explore, its still a no-brainer - just buy a bit less of every pack so you can make it through the full rotation.
tl;dr: I’d do it only if you need a disproportionate amount of Dark. Otherwise, spend some extra to try and even out the numbers and get some buffer, but don’t trait the troops unless you need them (either for significant star levels or to use). If you are dead set on finishing your arcanes this way (eg., you hate explore), focus on Epic glory packs when the are 2 for 400 and try to stretch your stash over the full rotation.
