Just a small comparison~
I rarely do my own calculations since I'm too lazy lol. However, there's very little data going around comparing the 4pc Millelith set to the standard 2pc NO-AP combo Zhongli tends to run so I just said fuck it and did them myself. So here's the plethora of values for comparison~
Assumptions are as follows:
- Level 90 Zhongli
- Level 90 Weapons (R5, but refinements do not make a significant impact)
- Level 95 Hilichurl to account for mob mitigation
- Level 6 Talents (Talent levels are generally linear increases in dmg, so expect roughly the same differences)
- 50/100 crit ratio
- 100% uptime on Millelith passive (possible with C1 or in tandem with Geo MC or Albedo)
Regarding main stat prioritisation:
For pure support Zhongli; run 4pc Millelith and Triple HP Mains
For anything else, go the standard HP or ATK Sands, Geo Cup and Crit stat Circlet (The dmg difference on his burst is very minimal, so pick the sands that has better substats)
To streamline the calculations (and cause I'm lazy lol), I'll just include numbers for the latter build as that's the majority of what people run or will want to run on him.
Weapon Choices:
I'll be using his most popular and optimal 4 and 3 star weapons for comparison.
Keep in mind that Blackcliff and Lithic will outpace Favonius if your Zhongli is invested in ER or is in a double geo comp with a C2 Ningguang carry, Geo MC and to a certain extent, Albedo as well.
Any 5 star will be preferred, with Staff of Homa being his top followed by Jade Spear, Vortex Vanquisher and *Skyward Spine respectively.
*Deathmatch outperforms Skyward Spine on average, but is still valued thanks to its ER substat. If you are running Zhongli in a non-double geo comp and find it hard to regen his ultimate, it's a solid option. Skyward Spine is basically Favonius Lance but with 5 star base attack.
Separate tables will be added to show the values including his 20% omnished and Geo Resonance
I haven't included HP% and ATK% substats as they're linear due to their additive nature. These values just give an overview of what you can expect.
All values shown are CRITS
tl;dr: Initial findings sees that there's between a 5.75-8% average total difference in Burst output going from 2pc NO-AP to 4pc Millelith. However, this is offsetted by an average of 2.25-4.25% loss in resonance damage. The biggest upgrade would be in shield absorption, gaining a sizable 38% increase if you opt for 4pc Millelith.

My personal opinion: If you've already invested into a good 2pc NO-AP set, I really can't justify spending resin in investing another set specifically just for Zhongli if 1. You already have him heavily invested and 2. Have other units to build. However, if you are farming for the Pale Flames set (which is very good for Crescent Pike phys-carry Zhongli) and good Millelith pieces drop, I would consider running it since the teamwide 20% ATK buff is solid if you can maintain the 100% uptime (it's basically a Noblesse buff on steroids) and the big shield is a nice addition to have.
If you're curious about the difference of going from 2pc NO-AP to 2pc Millelith-NO, there's an average of 1.4% difference (in favour of 2pc NO-AP) in Burst, slightly lower resonance damage compared to 2pc NO-AP but obviously you get a bigger shield (Refer to the second Edit).
Edit 1: Newly acquired Zhongli players are recommended to farm for 4pc Millelith, as they wouldn't have a good 2pc NO-AP set to put on him unless they pre-farmed and weren't willing to spend extra resources and resin into farming the new dungeon.
Okay, onto the numbers:

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If you want numbers without Geo resonance; It's about a -16-17% decrease in output~
Edit 2: Apologies, the shield values on the 2pc Millelith-NO combination is wrong. I forgot to exclude the 30% shield strength bonus from it. I'll update the values here:
Black Tassle (Shield): 9694
Deathmatch and Favonius (Shield): 8274
Not as much of a difference there in terms of shield values, so pick whichever set has better substats.
Okay, I'll probably never do another one these again, but if you guys like these comparisons, I "may" do another one in the far, far future when I feel like it lol.
[emoji]

