Which old posts and why do you doubt their relevance?
The ONLY way to get Narcissa’s Mirror is with Shiny Pet Charms.
Shiny Pet Charms are mostly earned from Legion WQ.
I’m not sure how effecrive Boon of the Zookeeper is these days, but you could try it.
You can also get some from Draenor dailies, but not very many.
Even as Level 1, you don’t have very many pets, nor the ones you would necessarily need to defeat Legion battles.
You will need Level 25s.
Here is a guide to the various ways to get your first pet to 25: Guide: How to get your first pet to 25
Note Approach 5: The Dragonflight Lubbins quesline. With any level 70, that will get you a couple of level 25 pets in an hour or so.
Once you have at least one Level 25, you can buy and learn more from the auction house.
After that, https://www.wow-petguide.com/Section/16/World_Quests gives you lots of strategies for defeating Legion Tamers to get Shiny Charms. From that list, you can decide the pets you want to buy or otherwise obtain.