When it's SAFE to do so you should be rocket jumping ontop of people and then following up with Explosive Shot and an autoattack. When it isn't safe to jump on them just Explosive Shot and autoattack. Those cover harassment.

When you're trying to gank someone wait for an opportunity, i.e. them being stunned or near half health just to be sure, and then jump on them, hit explosive shot and right click on them, hit exhaust/ignite and then hit buster shot so you push them into a wall or towards your teammates, then hit rapid fire and start autoattacking.

If the jump damage, the damage from explosive shot and buster shot AND the additional damage from Sheen/Lich Bane doesn't kill them instantly then autoattacking them with ignite on will more or less guarantee a kill. You have to think about dealing your damage as a burst, rather than like a DPS Tristana.

You are also correct that late game, past the 40m or so mark, this build is not nearly as effective. When you stop being able to outright kill enemy casters and carries, I'd advise you sell everything and buy as much of your physical DPS build as you can, you'll be more or less as effective as if you had bought all of it in the first place, and you can continue helping your team by dealing lots of damage to threatening targets.