Your expectations for toddler sleep seem to be off. 18 months is absolutely normal to still be waking up. According to the Lullaby Trust, 92% of 2-year-olds still wake up once a night and need resettling, and one of the big baby accounts found 91% on a big Instagram poll, too. Most aren't sleeping through - and that suggests that most sleep-trained babies aren't, either. I've got three friends who sleep-trained, but it didn't last for any of them. Perhaps some people fair better.
You can't say that your DS wouldn't have learned to self-settle, though, because you expected him to be able to sleep through at 18 months. He is the exception, by far, if he actually sleeps through now, and doesn't wake up and just not call out for you.
Your DD sounds like she has much lower sleep needs. I wouldn't worry about the length of wake windows, they're not very scientific. She needs the sleep that she needs.
My nearly three-year-old has started waking once a night in the past five months or so. Before that, he woke a lot more. He always needed me to settle him. He was out of a cot at 14 months as he could climb out - he stays in bed and calls for me. I can leave him now listening to his Yoto, but he would never tolerate the leave for a minute, lie him down and say goodnight type stuff, and I could never leave him to cry. I'm glad I didn't. I was brutally tired some days, but I did what felt right. He stays in his bed until around 4am most days now, then runs through to me for a cuddle and snoozes until around 6am. I love the cuddles. He's never taken himself downstairs or to play in the bathroom or whatever.
If you're going to go the CIO way, which some people do, there's nothing kind about any of the approaches. It's just going to have to be something you suck up to get to your end goal.