Now that I've created a simulator to model the accessory enhancement process (Accessory Enhancement Calculator), we can work on optimal enhancement strategies.
Point 1: Conserve gems by using the fewest number of batches possible.
Point 2: Make the best use of fodder resources by using all fodder available (rarities and elements).
Point 3: Conserve gems by using the highest rarity fodder possible, and using the rest to level that fodder (since LV1 synths are nearly free).
Point 4: Conserve gems by rear-loading high-value items into later batches, lowering the base level of each batch as much as possible.
Based on my runs of Rank 4 accessory quests this week, it looks like we can expect ~6 SR drops for every 1 SSR. That means we'll likely be relying primarily on SRs for fodder, at least if you want to have 9-12 maxed SSR accessories anytime soon. If you rely on SRs only (assuming a 1:5 ratio of same-element to off-element), you'll need to spend about 160k gems. If we mix in a 1:6 ratio of SSRs to SRs, we can lower that to ~120k.
By my estimates, that puts us at 1.44 mil gems for a full set of SSR accessories, which we can farm fodder for at a rate of 1x 120k accessory per 8 days. That seems doable, though it'll require some gem farming.