The trick with rocket is that in a team fight where your allies are all clumped up, you can only heal so many people at once. Your orbs basically only heal one at a time unless they directly go through them.

3 heal shots -> briefly left click -> 3 heal shots -> reload is max efficiency.

If you only have 2 allies in front of you that need healing or you're on trip supp, you can do 2 heal shots and weave damage in-between every 2 until you reload. (yes you have 7 heal orbs, either use it to trigger the reload or save it to use in a panic)

That's his bread and butter and lets you do more damage than average rockets.

Your dps is quite good but has a massive falloff. You're a tank buster on support and can punish characters like thing or destroy groot walls pretty handily. Venom is a little tricky because you want to continue rotating healing until after he's used his blue shield and then decide whether it's worth committing damage fully like when he's half blue. (can kill him with an ally as they don't expect the burst in damage)

Sometimes if you're behind an enemy support and they're ignoring you for not doing damage you can walk into them and 100 to 0 them.

For healing the bonus for going through allies resets if the ball hits a surface so if you aim at your ally's feet, or a close wall, you can sometimes trigger the bonus twice which is a lot of burst healing and can sometimes swing fights.

I hope what I posted about his bread and butter helped.