I don't know if this will make sense to you or not, but I will give it a whirl. I rebuilt a large portion of my docks, saltwater mind you, on the main channel in Port Isabel. Getting anyone to do the work at all down there is rough.
What I did was get pieces of of hard plastic water/sewer/culvert drainage pipe. What I got was kinda like the link below. I went to a local place that sold large sections of this kinda of stuff and in talking to them they ended giving me several scrap pieces that they had laying around as they were happy to be rid of it. The stuff I got was stiffer than what I am linking.
https://www.tractorsupply.com/tsc/product/culvert-hdpe-18-in-x-20-ft-hdpe-1820I then used SCUBA to get in and attach chains to the old pilings and with a comealong along pulled them out. On some of them I had to jet around them to loosen them up. The way I jetted was I used SCUBA tanks with weights strapped to them and would go down and turn those tank nozzles right where I needed to blow the bottom out. It took me about 4 or 5 tanks to do about 7 or 8, 8" old wooden pilings.
I then cut pieces of the pipe I had gotten, I used a saws all and cut it with an angled point at the bottom. Again on SCUBA I jetted these in while working them down into the bottom, I probably went in 10-12" below the bottoms surface. I then cut the pipes sticking above the surface somewhat evenly with a sawsall.
After that I got treated 4X4"'s and 6X6"s and cut a point on them, I want to say I used 8' ones on the deep part and 6' on the ones closer to the bulk head (where I used the 4X4's). I forced these down in the center of the plastic sections of pipe using a sledge, probably drove them down 2-3' into the bottom. Once I had them all in I ran a level line and squared them up and cut them off. I anchored/leveled the wood pilings, to the sides of the plastic pipes with fishing line.
I then orrdered a concrete mix, I forget what kind but was told it would work and set inside the pipes and to the wood (and it did). I filled the first pilings one Saturday, and then mid week build the dock out. The next Saturday I did the rest and the end of the dock. About 3 or 4 yards of concrete.
Obviously I am omitting a lot of details and minor problems and glitches that came up, but it wasn't hard at all just took time and planning and lots of barnacle scrapes. I have since helped a neighbor replace some in the same fashion.
Given that you are in fresh water I am not sure if you have wood worms or not. If you don't, I think you could drive some 6" PVC pieces in and put a treated 4X4" inside them and drive it in with a sledge. It took some time driving it in as you can't hit the wood directly, you have to use a thick shim.
Importantly a key part of this is to loosed the area up where the pilings/4x4's and such will go with pressure. I am sure a pressure washer will work, but I had SCUBA tanks and that air pressure when applied directly cuts a hole like a knife.
All in cost, and this was about 15 years ago, was about $1,500 or so. I have access to free SCUBA stuff so that made is simpler. The quotes I had gotten back then for removal and re-driving pilings was like $8,000 and Lord knows when it might have ever been done. Dock is still holding fine today and has been through 2 hurricanes, one pretty substantial.
