Hi Grant, welcome to the wonder of SUP!
It sounds like you have a great idea of how you want to enjoy this sport, and it is totally a good plan!
I came into it from the surf side, so my SUPs were all purchased with surfing as the only goal.
I discovered the beauty of flat-water after a torn muscle kept me out of the surf for a while.
So... I started paddling the calm estuaries and marshes around Cape Cod.
I fell in love..... a completely new world opened up to me, right in my backyard.
I am a big guy.... well, actually getting smaller as I write this, but when I started surfing and paddling SUPs I was north of 120 kgs.
I paddled my 9'6 surf SUP all year long.
Average outing was 6 miles.
I never gave a hoot about blasting down the middle, all worried about my average speed...... I dinked around, up and down interesting side creeks and stopping a lot to explore, or just sit on a rock and listen to nature.
Everyone told me I would get bored... that I needed a 12'6 raceboard, so I could go fast.
Pretty soon, I was paddling my 8'10 surf SUP 6 miles per outing.... even in blizzards..... and enjoying the heck out of every trip.
Funny thing is.... after 8 years of paddling the shallow edges, I am still awed by the nature I witness on every trip.
But many of the "fast" guys saying I would get bored.... faded from the paddle scene.
Bottom line.... I have always been a huge fan of smaller SUPs for flat-water.
Easy to load, lug and store
They fit in small creeks and tight marshes, but still fine for long straight paddles.
I did a video once of paddling a straight line for a mile, on an 8'10 surf SUP.
Full disclosure: I am part of the Sunova family now, because I love the people and the products.
Here is my full review of the 9'5 The ONE
Showing surfing, but the entire review was done during a winter flat-water paddle, in full wetsuit.
I was 109 kg at the time, so a LOT heavier than you folks.
When I handed it to my students, they love it..... both as an effortless paddler and easy to load on or in the car.
It is 32" wide, with full enough rails to give a new paddler a comfortable learning experience, and good enough design to ease you into a lifetime of surfing, once you are ready.
Very light for its size and price.... extremely durable
It comes in larger sizes as well... but I think this size may hit all your needs.