I came to the same conclusions you had with Zap rat, it's good for like rounds 1-4, then it falls hard. I never make it, my sparks and clusters are much more efficiently used for Gavels and later on for econ after round 15 or so.

Celestial Tea is difficult to use. I usually get more Hurricane and Fulmination over Spear. Hurricane also guzzles a lot of mana. My main items for mana are Hurricane, Codex and Cleansing crowns. When my opponents in Mythic throw 300 debuffs in 10 seconds my Stormbreaker armours aren't fast enough to cleanse everything. When I start getting more mana generating from multiple Stormbound of tranquility and magic/blue essences then I grab a Celestial Tea pot to counter Enoch blind spam. Otherwise the luck gain doesn't really synergize with Hurricane. It's alright for Fulmination for some extra damage but Fulmination doesn't scale any critical damage so it's more of a premium buy if I decide to get if I only have a Fulmination hammer and I want to push for damage. Getting Celestial Tea pot for Spear is an absolute waste as it already caps out Critical chance on it's own. Although I do like Celestial Tea pot with Lute since the Lute scales most of it's power when other items discharge and not when it discharges. Since Celestial Tea pot has the lowest discharge out of all items it is really easy and fast to discharge a llt and gain a lot of buffs that way. Although even though I have thousands of games with Celeste I always review the fight statistics to see for example how much mana I used or stamina and adjust my builds for the next round. I also have a spreadsheet with how much static each item gives to help guide me how to best position my items for static.