I dont know what to do in ranked anymore.(help from diamond+ players)

I've been platinum for about 3 seasons, If I don't hit diamond this season, it will be 4. I've learned to control my tilt, which is why I'm plat1 currently at 40lp. My biggest issue is this is just where people don't care. For example, last game a kass went 25/3 because my graves and draven didn't listen, as i told them not to roam since he was so fed. The main problem I'm having right now is that.

I will admit, most of my champion pool just isn't viable anymore, viegar, talon(whom i have almost a 70% winrate with 70 games) azir. and a few more. People just know how to punish a earlygame champion, and it's very hard to play those champions and do well, for example as talon I get zoned very bad. I'm just not sure what to do anymore, It is partly my teamates fault why I'm not diamond, but moreso mine.

The issue is learning a good mage, or assassin will take a while as I've played all these champions for years. Talon being the champion to carry me from bronze, to plat. What are some general things I should try to climb the last step of hell? I really enjoy champions that are hard and have good outplay potential, which azir satisfied that, but he's only viable to top tier players right now.

Anyways, is there any advice you guys can spit out? I've lost the diamond promos a few times now, and I'll most likely take a few days off, I'll be reading anything you guys give!