Launching any artistic venture is a highly risky strategy if you are seeking income rather than internal reward. There are many people in the industry and the distribution follows the power law - a very tiny fraction will be very good and earn significant income, and a large segment will get little business and earn little to no income. Discouragement is almost inevitable unless you have some significant specialist advantage in knowledge or experience.
Approaching a professional architect again will likely not be successful unless you have that special thing that will help to raise the team professional offering rather than dilute it. The "special thing" then becomes your product offering, while the garden design is a nice addition for the customer even if it is the main motivation for you.
One way to start, in addition to establishing a reputation on sites such as this one where you can demonstrate your background knowledge and skills under your own recognizable name, would be to take some of your best work to date neatly rolled in a tube and show it to landscapers in your area. Generally these people have investment in capital equipment to get the final job done and produce designs at the customer's request. Some customers don't need a professional landscape architect and will be content with someone less prominent. The thing about the landscape business is that is it highly seasonal so in spring it might be handy to have someone who can take the overflow design work so they can get back to repairing equipment and keeping the business functioning at high speed. Someone skilled and reliable who can take on the extras and help please the end customer could quickly move to an important position. That way you establish your name and reputation and could eventually become independent.