IMO there isnt a very tangible difference in champion skill. There will be people in plat who outperform Diamonds and vice versa. You are correct in assuming its the decision making. Now by plat you have most of the decision making down. Its really the little things that make or break a game. Being able to make these descions and act on them QUICKLY is key. Its ones thing to think hmm maby we can do baron if 15 seconds if we manage to pink it n then sneak it vs I know with our Mid Azir and %hp dmg from the jgler we can do this baron given a 10-15 second window.
I will give you some of the small things that set them apart.
• If youre level 9, UPGRADE your trinket. Only way you are allowed to not do so if you have barely enough to complete an item or wont be able to buy a pot and you need it to live. If you ever get to level 14 + and you havent done this then youre holding yourself back an entire 2-3 tiers.
• watching whether the enemy bot or top comes to lane first. Whoever came last means they leashed. Therefore enemy jgler is on that side so you have two options now. Counter gank on that side OR counter jg. If top laner came last but TP`ed in that means he didn't leash he did a camp himself. If you are laning you know which side to play away from and roughly where to ward now.
• Knowing when you can ward deeper into their jg and when you cant. That ward may not help you but it may save the life of another team mate if they are considering engaging or playing safe due to lack of information.
• The biggest thing that won me games (besides upgraded yellow trinkets) was me calling for baron. You have to realize the second you see someone on their team get greedy and go bot for a big wave AND they DONT HAVE TP then pressure baron. Often times the jungler will be doing a camp in that side. If the enemy blue/red is up and you know they will be going there then rush baron.
• Roaming when no one expects you to do so. I`ve picked up many kills playing swain lvl 4 because I pretend im going to go ward and just run bot and gank often leading to a two kills for our team and dragon. Dont do this at stupid times when the wave is at your tower because you will be fked for lane. Lower exp and gold and if ur gank doesnt net a kill your lane is over.
Some of these things you may know. Im not to sure what it is thats going wrong in your games so if you could give me more info I can provide a more tuned response.
Also doing these things doesnt mean you should be in Diamond. You should be doing these things along with a multitude of other things.