How do I get better?

You have to play champs that u dont really know very well in ranked. And yes, you gonna be losing , but that doesnt matter. Thats how you improve.

For champs i want to main in ranked, as well as my current mains, what i do is i played normals until i stomped with that champion 9/10 games consistently and that last game i still didn't do bad. Once I'm there I'm confident bringing them in ranked. Another way is to play the champs in ranked a lot, but you might lose a lot of LP for doing that.

Lastly, champs that are passive and offer wave clear from far I'm less worried about because I'm safe and shouldn't be feeding

I honestly feel like biting the bullet and spamming ranked is the best way to master champs. Get to the point where you're pretty stompy in normals with them first of course, but beyond that you're going to have to learn from ranked, and you will get punished, but that's how you learn. You might fall slightly in ranks, but you can switch back to your best champs for a bit to pull your LP back before returning.

Alternatively, if you do play normals enough, eventually your normals mmr will get high enough that things will be challenging. People don't take normals that seriously compared to ranked which is why normals are relatively easy, but if your ranked mmr is bronze-silver or so, then even random golds playing their off-role in normals are probably going to be harder matches than what you find in ranked. If you're gold, you might need to get up to a plat-ish level mmr for normals to be as competitive as ranked, and so on, but you can definitely get there.