Buying advice

-Look for reviews and buyers guides(avoid most tech media sites as they bias towards selling you the headphone with the affliate links prominent in the review)

-Try to think what do you like and dont like about what your current headphones has, try to find something that better in ways you want or try to find something that complements your headphones

-More expensive doesnt mean better quality, a lot of people would call ksc75s or insert hyped trained budget chifi IEM 70-80% of what you could get from headphones 50x the price. There are also kilobuck or pricier headphones that are either plain out bad for the price or very polarizing much like your meze 99s, which at the height of its popularity had people fighting whether its good or not in this subreddit on the daily.

-Try to satiate your upgrade urge by trying something whether it is a different genre of music, using EQ,etc

General financial advice (as audio for most people being entertainment and hobby for us) - Only spend money for hobbies that wouldn't hurt your financial situation if you just burn the money spent cause every hobby has the potential to be a money sink

Also to learn more about headphones https://headphones.com/pages/measurements-and-frequency-response

They have other articles for their site and videos on their youtube channel