Landscape architects and landscape designers are similar professions, but usually landscape architects have a degree in landscape architecture and are licensed while landscape designers are not required to have a degree or to be licensed. A plant designer usually works with indoor plant designs or garden displays instead of entire landscapes. Sounds like you would need either a landscape architect or landscape designer depending on your budget and the scope of what you have in mind. One thing that might help you greatly in narrowing down what you want and how you want it is to visit as many public gardens and other private gardener's landscapes as you can. Maybe someone from this forum, that lives in the Nashville area, could point you in the direction of some interesting gardens in your area. If you know any fellow gardeners in your area, ask them to point you to some of their favorite garden places or even give their input and ideas about your project. If I lived in Nashville, I would drop by. I am currently in the process of designing and building my own 25 acre arboretum. Now I look at most yard designs as far less challenging.

Need size of slab? What activities do you want to plan for in the entire yard? BBQ, dining, lounging, sports, grassy play?

For the backyard, instead of expanding the patio, try creating a separate "room". You don't need to add a building, just add a path to a place. If the room is in between places or heavily used you should pave or gravel it. If it's a private place to relax go with lawn. If you need privacy the quickest way is a fence or trellis. You can also set off a room with low shrubs or flowers, or rock work or sculpture.

You absolutely want to hire a designer, especially if you've never done much landscaping before. A local design professional who can visit the property, talk with you personally to assess your wants and needs and then convert that into a workable design plan you can implement as time and budget allows or hire out to a contractor is the most cost effective and expeditious path to follow. There are various ways to find a designer. You can start with the Houzz Find A Professional feature, contact the Association of Professional Landscape Designers (https://www.apld.org) for referrals in your area, ask at local nurseries or garden centers or inquire of neighbors with landscaping you admire. Other than some very rudimentary suggestions, you are not going to get a lot of help here. Most projects are beyond the scope of what can be offered online and from a forum that is populated by almost no qualified landscape designers. Most of the "pros" that post here are interior or kitchen/bath designer and the skill set just does not translate :-) Landscape design is much more than making a few simple suggestions based on one's personal gardening experience and most landscape designers have had considerable training. A lot of folks shy away from hiring a professional due to cost but it really is a valuable but modest investment, especially if the homeowners do much of the work themselves. Labor is the high cost factor in landscaping, not design fees!