What's the best way to play as a tank driver? Should I be reloading after every shot? Alternate between the shotgun cannon and the main cannon? I get caught with my pants down all the tank in a tank, as I seem to have to reload ALL THE TIME. That's a player mistake that I'm looking to correct.
I have more kills with the heavy tank than every other weapon in the game combined. I've played Conquest exclusively and if I get the tank I usually finish at or near the top of the scoreboard (and near the top in flag captures/defends). They are extremely overpowered (and probably will and should be nerfed eventually) but you will still get blown up if you are careless or allow yourself to get swarmed.
The main shell and buckshot draw from the same pool of 6 shots, which will fill back up on its own. You don't need to reload. I use primarily the main shell and aim at the ground just below or next to an enemy (or a destructible environment if they are hiding behind one). I use the buckshot mainly if someone is hiding behind something non-destructible and only exposing part of their body. If they are completely behind a rock or similar non-destructible object you may still be able to kill them or at least injure them by shooting at the ground next to it with the main shell.
You should be capping and defending flags as much as possible but preserving your tank should be top priority. Try to engage other tanks when they don't see you. If you can land 2 shots before they hit you and they don't have other players supporting them, they are done. Likewise, don't instigate a fight with another heavy tank if your ammo is still refilling. If you are taking damage from infantry that is swarming you, throw up your smoke and retreat. Backing up in reverse works well. Save your instant repair for when you really need it. If you are just slightly damaged and you have time to do the long repair do that instead. If I take a lot of damage, I will often instantly hit the smoke and instant repair, throw it in reverse, and start killing anyone that tries to chase me. Then I'll find a safe spot and repair. It's a long match and keeping my tank alive as long as possible (preferably the entire match) means I probably win the round. There's no need to take unnecessary risks.
When shooting at other tanks, there is an annoying issue with hits not registering sometimes. Try to land direct hits to the front, back, or the sides (or the top and bottom in rare cases). Glancing shots at an angle seem to be the most likely hits to not register. If you find yourself at a disadvantage against another tank and retreating is not an option, sometimes the best bet is to ram into them and disorient the other driver. If you can get them spun around so that you are aiming at their side then their main gun is useless and you can just keep landing hits on them. 3rd person view is probably easier for this.
Anti-tank mines are instant kills so keep an eye out for them. The behemoths will ruin your day, especially the air ship. The dreadnaught too if it can see you. You can help chip away at the behemoths with your tank. Just don't leave yourself over exposed. Be careful about parking your tank near friendly mines too. If they get detonated when you are near them you will get blown up.
I wish it wasn't so effective because it kills the emersion, but 3rd person view is extremely effective. It greatly increases your FOV and let's you see over objects. I wish there was no 3rd person view but I use it constantly and switch to 1st person to line up shots that are farther away.
You can take out airplanes in one hit. The big, slow, low flying bombers are actually fairly easy to hit with some practice. Even the little planes can be hit with a good shot.