Honestly, I find it rather easy !
The most important part seems to be not to let them over heat. You don't want your house to get above 80 degrees. And if it ever does, you need to take action, take the Crestie elsewhere or put frozen water bottles on the cage, etc.
So they do quite well in regular room temps. Mine fluctuates between low 70's and upper 70's. The lowest it got so far was around 68 and the highest 82 (power outage!! dangerous..)
Aside from that you want to mist them daily. You want the humidity to get high at night, but then be allowed to come down some during the day. Mine fluctuates between 50/60 to around 80% With some spikes here and there.
Most people do not provide UV light. I don't.
Diet is easy. Tons of pre made powders you turn into smoothies. Feed 3 to 4 times the week. (more often if they take it or are small) Those diets have everything they need. You can add bugs. Crickets or Dubia roaches. No meal worms. Feed your bugs well and dust them with mineral powder before giving to the Crestie.
In a new home they often don't eat right away, sometimes up to a couple of weeks. But they will
I provide a tiny water dish, some drink out of the dish, some drink droplets after misting.
Setup can be more basic or elaborate (bio active), but not to big, esp. when they are tiny.
Some are flighty, others more chill. They all calm down somewhat after the move, but some stay flightier then others.
Gosh, I managed to make that sound more complicated then it is, LOL. Its really rather basic. Room temps, daily misting, Crested Gecko Diet 3 to 4 times the week, bugs if wanted.
There are quite a few people on here that are very knowledgeable when it comes to Rhac geckos. Deborah, mleedee, etc. They will help you with any concern and question you come across.
About your other animals...rescues die sometimes. Oftentimes they are already quite damaged when you get them. Its sad..but its part of rescuing. Don't blame yourself for that..