Probably 90% of my brewing is one gallon batches these days. Next year I plan on doing more 2-3 gallon batches so I can peel off an extra gallon for souring or aging so I have a supply of beers to blend down the road. Don't feel limited by the size of the batch. You can brew pretty much anything at a smaller size. I have done all kinds of fruit additions, dry hopping, wood aging, souring, etc. on small batches.
If you already have the equipment from doing partial boil extract batches (particularly a 2+ gallon kettle) then you are pretty close to having everything you need to brew small batches. You need an adequate fermentor and a place to mash. I use those five liter wine jugs as fermentors but there are other options out there. Many people brew smaller batches on the stove top as BIAB but I prefer using a two gallon cooler because I never had stable mash temperatures on the stove. Either way, you are $20 or less away from a full all grain set up.
Brewing a recipe over and over to perfect it is a better strategy than brewing it once or twice each year and hoping you remember what you wanted to change but don't feel like you must perfect one recipe before you move on to the next. One gallon of beer will yield 10 bottles at most (if you tweak your system well you might get that 11th bottle) and often you will only get 8-9 after accounting for trub. That isn't that much beer. You could easily work on 2-3 recipes at a time with some other stuff thrown in to keep it interesting. However, I'd say brew some good recipes to get acclimated to your set up and then start working on recipe development. Your technique will improve after a few batches and you may find that some of the early changes you made to a recipe were really about your technique.
You will definitely spend some time with your bottling equipment with one gallon batches unless you want to buy one gallon kegs (which run around $100 each) or burn a lot of CO2 filling three or five gallon kegs with one gallon of beer.