Steps ive found to planning the most efficient route possible:
You'll start your route near the physically closest stop. First decision is if you end the route at the farthest, or make a loop and end back around the start. This depends on the districts, traffic, street accessibility. Not a big deal either way.
Carve your path out to get your businesses done. You can hit resis on the way but commercial is priority.
Avoid driving the same road twice. If you have streets and stops in the shape of an A then it isn't going to be possible, but avoid it when you can.
When you have a road you must drive twice, hit the stops only on the right each time you drive through (cul-de-sacs being the main example). You should almost never drive a road 3+ times unless you have pickups later or something.
About the left turn thing- in my experience, ANY turns onto main roads are something to avoid. Even right turns because you'll get stuck behind someone turning left half the time. In resi, I find that no matter which way I'm turning, it's clear 95% of the time so I don't avoid lefts. So prioritize getting your turns done in resis so you spend as little time on main roads as possible while delivering; main roads however are great if you're moving a long distance, it's rare you can get somewhere faster by traveling through an entire neighborhood with all the stopsigns, turns and lower speed limits.
As others have said, work in a loop. Plan out your movement between big districts first, then work out the intricacies after.
Finally, talk to dispatch to get your PAL order set up in the way you want to deliver. None of this makes sense if your first stop is PALd to 8000. I don't deliver 5000 and above until I have a very clean truck. Even anything x500 and above I avoid in the first couple hours, unless EDD is going in order correctly.