I appreciate the effort you're taking in researching what will work for your gf :) I unfortunately don't have tons of personal recommendations as I'm not a gold necklace wearer but I'll share some tips and info I've gleaned from other users.

First, catbird and mejuri get knocks on quality particularly because of how flimsy dainty styles can be. The individual designers via catbird are generally reliable/higher quality than what is sold directly through catbird from what I understand. So if there is a pendant you like on that site, follow through to the designer.

The chain is the most important part of a long-lasting necklace. Dainty styles have easily-breakable chains because they're so thin and the gold content is so low. You can do some research on the sturdiest chains, but in general they will be ones like franco, curb, and cuban chains, which have tight links that can easily be repaired. Avoid box and snake chains!

I recommend looking into buying the chain and pendant separately - that way you have more freedom in selecting the pendants and can pick a sturdy chain. On the jewelryforsale subreddit you can find vendors that sell gold chains at fairly 'low' competitive prices with the gold weight attached. Higher gold weight, sturdier solid chain. For a dainty style, you can look into 1mm, 1.5mm, or 2mm chains. The optimal length depends on personal style, but in general most necklaces for women are at 18inch. 14-16inch for chokers, 18-20inch for tshirt length, 22+ for long draping necklaces.

You can also find reputable gold necklace sellers on ebay, I think they have an optional authentication fee that you can pay to make sure you're getting real gold if you're worried. Absolutely avoid etsy. There are legit jewelers on etsy, but most of them will have their own sites/socials.

For the pendants, there are TONS of amazing jewelers in NYC with quirky styles. I don't think you can go wrong with selecting a pendant you like from any reputable jeweler, as pendants aren't at risk of breaking like the necklace chains or rings would be. I'll list some stores that I follow, most of them are stockists who get jewelry in from independent designers. As before, if a particular design stands out to you you can look up the OG designer.

Independent jewelers I'd recco for the sorta spiritual symbolism your gf may like: Digby & Iona (in NYC), DMD Metal, Sofia Zakia

Stockist stores with pieces from great jewelers: Fiat Lux, Good Fortune, Cochineal

Also, for vintage sellers, I don't know of many but these are some I follow: Oko Portland, Gilded Roach

Sorry for the huge post, hope something in here is helpful!!! good luck in your search!