Before I begin, this guide will not be great for optimizing plort output. In fact, it will be working directly against some strategies for maximum plort creation. If you want a lower maintenance ranch, you must sacrifice some plort production. I've still made more in-game money than I'll ever need for anything, but that earning will not be immediate.

A common mistake I see people make is trying to have too many of each slime. I recommend no more than two largos or four regular slimes per corral. This produces an adequate amount of plorts without overcrowding. Also, fewer slimes require less care and resources, and therefore less time spent on the ranch.

I also recommend fully upgrading all of your corrals (minus the solar shield if the slimes are not sensitive to light). This keeps your slimes content and safe.

Do not leave the auto-feeders on their default medium setting. Put all of them on low (turtle) so you won't have to refill them as often. (Your slimes will get plenty of food as long as there aren't too many of them, trust me.)

If you choose not to feed slimes their favorite food, you won't have to clean out the plort collectors as regularly, either. Of course, the downside to this is that they won't produce as many plorts, which means less money for you.

Fully upgrading your farms and coops as well will guarantee you have plenty of food to offer when the feeders do need refilling.

This system takes a couple in-game days to be sustainable, but once it is, it is easy to maintain.

Certain gadgets are extremely useful if you'd like to minimize the amount of time you spend on your ranch ranching your slimes:

Market and Refinery Links
I try to have one of each of these in every unlocked area of my ranch. This makes it so you don't have to walk all the way back to the market or the lab to deposit plorts. The walk will still be necessary before you unlock your blueprints and create them, of course.

Silo (not technically a gadget but I'm putting it in this section)
Having a silo (multiple, really) is useful in the event that the farms/coops produce too much of a resource, or if you want to hold on to plorts while you wait for the market price to be ideal. Either way, having a place for excess stuff will make it easier on you in the long run.

Scareslime
I have two of these placed right in front of my ranch to prevent the pink slimes outside from entering, stealing my food, and putting my largos in danger by plorting all over the place. Decreasing my paranoia about a pink slime ruining everything made it easier to stay out longer.

Slime Toys
Useful for the same reason as the music box upgrade. They help keep your slimes more content in case you're out too long by mistake.

If you follow these steps, you should be able to leave your ranch and explore or collect materials for gadgets for 3-4 full in-game days at a time without encountering any catastrophes. When those days are up, I spend 1 in-game day on refilling the feeders and cleaning up plorts. This is the best way I can think of to keep your slimes happy and plorting while you explore.

It's not fully automated like many of you seem to want, but, in my eyes, part of the charm of Slime Rancher is taking care of your slimes. Hell, it's the title of the game. So, I don't see 100% automation as a necessity for it to be enjoyable. But for those of you who find the game in its current state tedious, I hope this helps a bit.
Have fun ranching!