First of all, it sounds like you are on a great track for learning to brew great beer. Conversely, when I first started, I made a lot of kitchen sink beers, which generally suck.
As you say, there are really two things at work here with you being a newer brewer.
1.) Brewing good beer (notice I didn’t say “homebrew”…see my countless rants on this forum) is primarily about process. Good cleaning and sanitation, controlling fermentation temperature, yeast pitching rates, proper conditioning are all elements/facets that should be brought to a moderate/above-average level before designing your own recipes.
2.) Recipe design is about, as your LHBS wisely advised you, deciding what you want to drink/share, then working backwards. This applies to a lot of things, including cooking. In your case, your primary concern with recipe design is to learn about ingredients, which is a great way to brew.
I would decide what ingredient you want to focus on, then design the recipe around that. If you want to understand a malt, use some neutral/noble hops just for bittering. If you want to understand a hop, use a lot of late/whirlpool hops but keep everything else neutral.
A great thing to do to learn about process/ingredients is a simple recipe with one slight change, split into two fermenters. For these types of brews, I would focus on a process element at least in the beginning. For example, if you like American hop-forward beers, I would try brewing a full batch of Vienna/Centennial SMaSH, fermented with US-05. Ferment one in a water bath in your basement (I would guess +/- 60-65* ambient this time of year), and the other in a water bath in your upstairs (probably closer to 70*). I also used to make a big strong batch of wort (1.090 or so), then dilute with water before the boil down to 1.060 or so, and do two separate boils, each with different hops. Tasting them side by side really allows you to learn about the ingredients.
I would also recommend “Designing Great Beers” by Ray Daniels. Really an amazing resource for getting into the mindset of building your own recipes.
Keep up the good work!