I use a combination of the major system and random things.
You’re fine, the major system is useful because it isn’t as easy to come up with random 300 images, but the point is for them to be memorable, if your current list is memorable to you then you’ll be ok as long as you keep using it.
For example, my number 18 is Android 18 from DBZ, there isn’t anything about the major system in that image, it’s just very memorable to me so it works.