Well, I see some pro tips here, i still consider my self a newbie , But i Read and watch a lot.
ARAM n QM is hell different.
ARAM helps u get stacks where u need to hit heroes, Kelthuzad wil get stakcs up real fast, thrall lightining stacks go up real fast.
To start with, dont go the normal build, you should almost ALWAYS adjust for ARAM from build u mostly go in QM.
And what you say so far, you been comfortable in duels and 1v2 , but not mostly in team fights.
I had this problem too, I watched a youtube video on something like “Team Fight Mechanics” it helped me a lottt
heres the few points I learned from that and made some my own rules to excel in team fights (most of them r simple, but you dont tend to giv them weight unless u hav them in mind) as an assasin (i play assasins only, and i LOVE to hav a kt or li ming or greymane in ARAM)
1 = KEEP DISTANCE
When KT or Li Ming , ur the target. keep distance. DONT have to hit a combo everytime, u can just flamestrike n mov bak without putting a bomb, or put a bomb n mov bak without putting a flamestrike (for starters)
2 = HAV UR ESCAPE READY
dont teleport IN with Li Ming, (unless you going for the kill to teleport bak). Keep KT’s E ready to stun to run back.
3 = B BEHIND THE TANK
Yes, everyone knows it, but wht i mean, u got to keep one eye on the target n one eye on YOUR TANK. you see him mov bak even an inch, you mov bak too,
If you stay behind tank, n stutter step along his movement. almost all skillshots coming for you can b soaked by the tank
4 = GET GLOBES FROM MINIONS
You cannot avoid that, its a HUGE difference
5= DONT AUTOATTACK (as a mage)
Your autoattacks wont damage much, but if theirs a valla or greymane there, they can drop you in few autoattacks if you overstep.
FOR AA RANGE HEROES (Valla, Greymane)
1 = Stutter step. keep dancing, frustrates the enemy when his shots miss u n he does a mistake
2 = Dive in ONLY for the kill and DONT overstep.
3 = GET GLOBES (the one from minions) You CANNOT avoid them
4 = Initiate AFTER the tank, there is limited healing in ARAM , hearthstones have been disabled. Dont b Superman 
Overall, if you hav decent players to play with… You can excel in team fights by playing more ARAM style matches.