As for going brain dead, I play most games brain dead myself even though I'm the kind who prefers relying on game sense over anything else, so I'm just running on my passive game sense and style etc, but that works for me at this point in time (66% w/r over 80+ games, but don't do what I do). If you want to keep a presence of mind and not autopilot, I suggest vocalizing your thought process, and try to keep it simple so that it doesn't slow down your thinking. By vocalizing, you can just speak out loud. You don't need to shotcall actively; just muting your own mic and talking it out helps to keep you focused. You'll notice more of what you're missing out on etc when you start to verbalize them.
As for tanks, I play them all as a flex player so here's my take. The general role of all tanks is to create space. Pick the tank that will help provide the most space for the team given the current situation. While Rein and Winston are the core main tanks, in a pub, it's fine to not run either and go for another tank composition, so long as you feel it best suits the situation. Pub comp games tend to have weirder comps so you can justify the odd picks if it increases your chances of winning. Winston is generally a good pick in every situation, unless you find yourself being hard countered against a heavy damage comp that instantly deletes you and you can't rely on your team for protection or follow up. Reinhardt is good in more enclosed parts of maps where you can block most damage coming from a direction, and/or when high ground isn't that important. For example, the many last point of Payload maps tend to favor Reinhardt. Orisa is for cheese / damage comps. You can run her on pure payload attacks sometimes, especially so if you cheese it with Mercy + Bastion. She's the ultimate cheese and something you might want to pick if you really need to go all in to hold/attack. Zarya / D.Va are almost always off tanks. Zarya is better vs spam, D.Va is better at scaring away squishies and challenging high ground. Winston can fill the off tank role when played with Rein too. Hog is for pick / damage / shield break. He's okay to occasionally face tank for the team though I don't expect that to be that useful in lower levels since people probably don't rotate/pivot around damage mitigation that well.
Back to making space. Now, after picking a tank, think about what you need to do to create the most space efficiently. Do you go for a dive in, or do you stay and play around barriers? Refer heavily on both team comps, the map, and current positioning of everyone. Offensive jumps in to disrupt might sound good and simple, but you really need to be aware of what poses problems to you when and after you jump in. You should always plan to be able to jump in and back out before you die just on your own. If you feel that you require significant team support to clear the jump or you might not make it out alive, don't take the jump. Pay attention to what heroes can delete you quickly, and at the next level, their cooldowns. It's perfectly fine to never jump in if you feel that you can never get an advantageous jump out of it. There are some cases where it's fine, if not actually a good move, to dive in, get a crucial kill or break formation and die afterwards if it lets your team win the fight, but one thing at a time. By jumping, I mean it applies to both Winston and D.Va, and perhaps Rein to some extent. Figure out who's the highest value target you can kill, and who is the easiest target for you to kill, and factor in the ability to escape after the jump. You'll get used to the decision making after some time.
If you can't jump or have no reason to jump them, what I like to do is to save my jump CD to counter jump, i.e. jump after their tanks jump. This is especially true for D.Va. D.Va Boosters can be used to boop away a Winston jump, and is useful to chase an enemy D.Va beelining for your teammates. Doing a short Winston jump is still useful for burst damage and knockback, or use it to go straight up and down when you're reloading or low on health. Rein v Rein you can just cancel their charge assuming no Zarya bubbles. Or if you're feeling cheeky you could charge when / before Winston lands to pin him.
Can go on forever, but this is the gist of it. Hope it helps you.