Apart from sliders, one thing I find useful is to make an effort to consider what unconscious rules I might be applying to my sims when making them. Then try to break those rules. It can be hard but once you know what ideas you have in your head about how a sim needs to look then you've identified something you need to change if you want sims to not all turn out the same. After you've made sufficient numbers of sims it can be difficult to find ways to make them different. I have kept hundreds of my sims and use them in my games that I often turn into YouTube videos. I love having the populations of my towns having varied appearances and I love using CC skins, hair and makeup to help me achieve the different looks.
Here's a few examples of what I've come up with over the past few years. I've been making sims since 2009 so I've made a lot of them, these are just a few I've made relatively recently. Sometimes I go on Sim making binges - just making new sims that I have no need for because I just want to play at making them look different. Then the challenge is placing them into games as I don't like wasting them.
Brian is an old sim from several years ago - before Sims 4 release
https://i.postimg.cc/8CLw8Kzf/Brian-Greene001.jpg
Crimson is one of my most recent creations
https://i.postimg.cc/fLPkqpRX/Crimson-Flower001.jpg
A recent household I made recently just to see if I could make some unique looking sims. Now I've added them to a game but haven't spent much time with them yet.
https://i.postimg.cc/DZBdH8QX/MFIBS011.jpg
Katie is an old sims with new hair and makeup
https://i.postimg.cc/Tw22z9Tb/KatieKingston003.jpg
Here she is in her old hair and makeup
https://i.postimg.cc/c4PHvwcq/KatieKingston001.jpg
Two Sims I made a year or so ago - they are in a video series
https://i.postimg.cc/V5yJSntp/Caroline-And-Wilma002.jpg
A composite pic of one household I use in a video series. Most were made a couple of years ago but three are several years old but given makeovers.
https://i.postimg.cc/NMBjVqq2/Team-For-Egypt001.jpg
Nate is one of my favourites.
https://i.postimg.cc/B6M1FNF7/Nate002.jpg
Fay is an old sim with new hair
https://i.postimg.cc/9MJnmVsn/Fay005.jpg
Mike is different
https://i.postimg.cc/Hx604T5g/Mike001.jpg
A few I made on one of my Sim Making sprees
https://i.postimg.cc/qRjTDyhp/Sims009.jpg
https://i.postimg.cc/JnPQy4gT/Sims007.jpg
https://i.postimg.cc/hj2TMZKM/Sims011.jpg
https://i.postimg.cc/FRTWXRsb/Sims004.jpg
https://i.postimg.cc/cHDSk0p2/Sims002.jpg
https://i.postimg.cc/nh3StDXz/LouisEgar008.gif
Some of them have made it into games but most haven't yet. There were more made at the same time but I think you get the idea. When I make a lot of sims at the same time I try to make sure each one is different. I don't try to make any of them beautiful. I just start fiddling and gradually I get an idea of what I want them to look like and let that inspiration take over.
Have fun and Happy Simming