Baby steps... what next?

I would add 10% whole wheat, 5% whole rye and 5% whole spelt flour to the dough flour and remove a like amount of the AP and up the hydration from 71% of the 1:2:3 to 75% for the 20% whole grain version which will be be more thirsty.

This will keep you in your comfort zone but give you better flavor overall.  You crumb looks great so this is a good next step.

Welcome and happy baking 

Looks like you've got a good routine down, and lovely looking bread - well done! As dabrownman says, try different kinds of flours now. Then he and Lucy could teach you a thing or two about simply amazing kinds of starters to change the flavour! Maybe that's for another day though.

Find some add-ins that you like too. One of my favourites is sage and onions. Or olives and rosemary. Roasted garlic and parmesan.

Of using different kinds of flours and add-ins. Once you've got a good working method and bread formula going, then's the time to see what you like in a bread.

30% whole rye is nice as well, as is a similar percentage of malthouse type flour. Raisins are really nice in a dark rye bread.

As for salt, aim for around 2% - it's easy to remember :-)

Buy this book:  DiMuzio's Bread Baking.  It's a great text book about bread baking.  It'll teach you about bread baking moving from start to finish.  It has all you'll need as a beginner.  It's inexpensive and, better yet, can be purchased on line at Powell's Books or Alibris used.

Working your way through the DiMuzio text will give you expert advice available to you quickly, even when your internet connection dies and it won't leave you wondering whether your questions are being answered at TFL by experts or not.  It's like having an expert at your fingertips.

If you want a much more detailed text, look at Hamelman's Bread.

You're making some good looking bread, that's for sure. Since you're new and looking for baby steps, I'd suggest keeping your recipe the same, but manipulating your starter and dough hydration in various ways so that you can learn how both can affect flavor, crumb, shape, dough quality, etc.

By keeping your basic recipe the same, but just altering those two variables you'll really be able to pick out the differences each one makes. That will help you build a solid foundation of technique from which to branch out later on.

Regarding starter: you can try using more or less starter while adjusting fermentation times to compensate. You can compare the flavor of young starter vs. mature starter. Stiff vs. liquid. Old dough vs. fresh starter. Once you really begin to understand how starter manipulation affects the qualities of your bread it opens up a whole new world of opportunities for flavor adjustment.

Regarding hydration: pretty simple here, just mess around with different hydrations and see the results, especially in crumb. Gradually improve your ability to handle wet doughs by increasing the water bit by bit -- it's a lot less frustrating than just jumping right into a wet sloppy dough.

Simply getting a solid handle on those two aspects of bread baking will really set you on the right path.

Cheers!

Trevor 

I suggest you start by partially substituting one type of flour so you can see, for example, what 10% rye does to change both the dough and the baked loaf. Then, you can try 10% whole wheat, which will let you see how this changes the dough compared to both your original all-white loaf and the 10% rye one. It will also help you to decide what to try first when you start using a mix of both; i.e. whether you want to use more of one or equal weights. 

You should also try each flour but at different percentages in order to learn how 15%, 20%, etc. compares to 10%. 

Fwiw, after my first few white loaves, I started my own experimenting by using multi-flour blends, but soon realized that this made it harder to learn about each one. So after a few loaves, I "went back" to the type of step by step approach I've outlined. Of course, it's not a "one size fits all" type of situation, but it was a better fit for me than the direction I initially took.