You mentioned that all you have is a liner. Did they put any sand on top of the liner? Former Aquatic Science teacher/TX Stream Team Instructor and I consult friends, neighbors, ranchers, and former students on pond restoration and maintenance.
I hear that you don't like the Johnson grass, but nature without mowing and chemicals would naturally create a buffer zone with grasses and eventually aquatic and semi-aquatic species that act as a filter/sponge to prevent massive amounts of silt, harmful runoff chemicals, fertilizers, manure, pesticides, etc. Everyone wants to have it mowed clean all the way down to the waters edge, but leaving a zone of unmowed grass as a buffer zone would be beneficial in the long run. If not already done, I would create a sandy shore zone so you will be able to move comfortably around your tank to fish and also act as a second filter after the grass buffer zone. Pea gravel & sand in shallow flat areas would help with spawning in the future along with surface area for beneficial bacteria populations to grow. All water needs movement for proper O2 levels and proper circulation to remain healthy, stagnation is a killer! Windmill aeration, solar fountains, septic air pump aeration with weighted loops at the bottom, or in design you can create drop-offs, fake creek beds to allow the density of cooler sections of water to circulate the water naturally.
Lots of variables, topography, cattle/livestock, neighbors, soil types, natural vegetation, wind blocks from trees, your watershed drainage, etc.

You are at the beginning, I know you want it to have trophy bass ASAP, but patience is key! Chemicals are a quick fix, may work great in the now, but bad in the long run. Flowing water is healthy water. Need to put some sand down on the bottom so whatever you choose to put in your tank for structure doesn't eventually tear a hole in the liner.
Sounds like a fun project!
Where you located? May be able to give you some free advice if you're close by to help you out.
I can definitely test your water quality parameters

Here's a link to my LinkedIn:
https://www.linkedin.com/in/stephen-christopher-nolen-99a14824?utm_source=share_via&utm_content=profile&utm_medium=member_android

Also, if I may add; as mentioned above, a healthy pond will have a veg transition from upland, to semi-aquatic, to aquatic plants as you get down to the water. While it may not be as pretty as being mowed all the way to the back, it is WAY more beneficial to the health of the pond.

I offer you a different solution to your problem of having clean banks for access/kid fishing. In a pond like yours, where you are adding structure/habitat to a bare pond, this could be a great set up for you. Instead of making the bank 100% mowed/walkable/fishable, make 4/5 fishing stations. Think about it like a sporting clays course. right at the waters edge, make a small 4'x6' deck or platform with a mowed path to it from the back. if you scatter several of these around the bank you can have access to cast to most of the pond. Strategically put structure within casting distance of each. it will require a bit more cost up front, but long term maintenance will be WAY less.