Is this the Cabin Max bag you're referring to?
If so, the details say that its dimensions are 17.72 x 14.17 x 7.87 inches. Rounded up and down that's 18 x 14 x 8 inches. The side straps can cinch it down, making it smaller. The shoulder straps look comfortable and the detailed pictures don't say complicated to me. I like that it's RPET, polyester made from recycled PET bottles. This review from Margarita on 11/22/2022 caught my attention due to your size. I've edited it some. There is a mesh pocket on the side, but maybe her bag is an older version that doesn't have one or maybe the pocket is too small for her water bottle.
"This was exactly what I needed... . Very roomy. I didn't fill it all the way thinking it might not fit under the seat in one of the small United Express airplanes, ...but, I could have packed a lot more in it. The straps on the side are great for adjusting depending on how full the backpack is. I'm short (4'11") and have back and neck issues and wasn't sure if the backpack would be comfortable to carry, but it worked great. ... The front pockets worked well for phone charger, wallet, medication, and other items you need handy, even a water bottle. My only "complaint" about this backpack, is it doesn't have a mesh pocket on the side for the water bottle... . Overall, I'm happy with it ... ."
I have an Appenzell bag with 23 liters (1400 cubic inches) capacity. Note that the capacity listed is for the "main compartment" and doesn't include the pockets. Although I find it comfortable up to about 15 pounds, I find the multiple pockets unnecessary.
I also have the the first bag that Mister E linked, the Rangeland Travel Backpack. Newly designed for 2022, it's 15 x 11.81 x 6.89 inches and has a capacity of 21 liters. It's the simplest carry-on I own and has a trolley strap, backpack straps and side cinch straps. Still pictures showing a woman wearing it are accessible by clicking on the video. Amazon lists the 2022 versions and a variety of older ones (like mine), seemingly by color. Choose a light color to make it easier to see the details in the images, regardless of version. The shape makes it very easy to pack and I'm always amazed at how much it holds.
I learned last summer that the shape of the bag can be more important to packing than capacity. My smallest roller bag, with European dimensions (20 x 16 x 8 inches rounded up), turned out to better accommodate what I wanted to take (in Eagle Creek compression cubes) than my other less square international bags.
The changes you suggest for the Appenzell sound good, but you can easily take a look at other possible options readily available for the Frontier personal item limits. "Personal Item. One Included. Size: 14"H X 18"W X 8"D including handles, wheels and straps. Think purses, totes, computer bags, briefcases, diaper bags and kids backpacks."