Hello Flourgrader,
I wouldn't add another memory kit, it won't help with gaming performance plus you're just asking for problems by combining memory kits. If you want faster memory, you should get a whole new faster 16GB-32GB memory kit.
For the fastest memory, go with a 16GB (2x8GB) kit, somewhere in the 4000MHz range. 16GB is enough for gaming and is all I use, see my specs.
For 32GB which is more than enough for gaming, go with say 3600MHz.
For gaming, you want speed not capacity, I wouldn't even consider a 64GB memory kit for a gaming pc.
G.Skill Trident Z is a popular brand, they come in RGB too.
https://www.newegg.com/g-skill-16gb-288-pin-ddr4-sdram/p/N82E16820232443?Description=trident%20z%204...This memory is on G.Skill's QVL list for ASUS Z370 and Z390 motherboards.