I think your definition of a project is just fine, and I often have several parallel next actions also.
The important thing is to keep an actions list in your project support. This includes all the possible actions you can do to move the project forward.
So you would put the actions there - I should loosen the soil, buy a hose, buy some plants, start some seeds, get some articles on using seasonal produce
From this the question is what should I do next? Check for logical sequences (eg prepare the soil before buying plants), this will make some actions come later. As you are feeling overwhelmed don't feel you have to put all of the next actions, you can choose to focus on one area of the project if that makes you feel better and more focussed. You might choose to just focus on setting up the garden beds and buying equipment, and only do next actions related to that. Then when that stage is finished, then start buying the plants and seeds. It could slow down the project but if you are feeling overwhelmed it will also reduce your stress.