Alright, I managed to get every last agate from Dragonspine - my tree's up to level 10 now, and as Serenata said I'm pretty sure that's only possible if you collect all the agates from the map and have done all the Crimson Wishes so far. The feeding the foxes and achievement was cute. I haven't crafted a Frostbearer, mostly because I didn't have a spare Catalyst Prototype, but it's probably not that big of a deal anyways since there isn't a Cryo catalyst user yet. I'll probably end up crafting one eventually anyways, just so I can say that I have one.
Speaking of the new weapons, I ended up equipping the Snow-tombed Starsilver I found onto Xinyan. The weapon has a physical dmg bonus substat, which goes well on her despite not being Cryo. Plus I managed to roll Qiqi from the Standard wish, and her skill's constantly applying Cryo means that the weapon's damage bonus works fairly reliably anyways.
Also, it seems like they're considering buffing Zhongli even more - according to this post, in addition to the previous buffs listed his shields will now reduce the physical/elemental resistance of nearby enemies by 20%.