Ok so ignore all the team stuff and look at the individual personalized advice that I tailord just for you.
Your playing rein, people standing next to you shouldent get sniped. If your stabding there with your sheild up and not shuffling around blocking shots your not doing anything. Theres no point in sheilding if your letting your team die behind it.
If you want to carry, you dont need to be DPS. Nor do you need to be hard careying to push throgh plat, you just need to do slightly better then other people in simmialr heros.
Its good that you play a diverse group of tanks, I recomend mastering the winston punch/jump combo and watching XQC if you want to get better at winston, who i think is better for you to climb if your team is bot helping you.
In addition you really should have a DPS you can play well eaither way, incase your team needs one.
In addition, you said that theres rarely 6 in team and thats something that is just not true in plat. Sure you get one or two stubborn dos mains every coupple of games but they tend to join if you talk to them positivly in team and say you will support there hero choice.
Finaly, from everything youve told me you sound like you play far to passivly and I thibk you should try throwing in some agressive plays, even if they fail at first keep practicing and find your limits. One recoomendation for agressive rein play below mid diamond is that if you jump and look down while facing someone with your sheild up while there trying to walk into your sheild they usually miss-time there shots and cant hit you, causing heros that would normaly shred you like reaper and hog to have to break your sheild first.