I should add that we live near a small river/stream and have a septic tank so I have nowhere to really dump the chlorinated water after 24hrs of shocking. 2 years ago I just emptied everything into the septic tank but neither is ideal as emptying it into the land results in it entering the river.
This is just talking about the sanitizing.
You are right about the septic. I will be very careful to limit how much goes into my septic tank. I will empty the well to the ditch. I have a small utility pump that I expect to press into service for when I drain the chlorine from the water heater. I could just use well water to flush that through, but it seems to me that running the maybe 50 ppm water I plan to fill the WH with could go directly without doing a longer mixed water flush. I may or may not pump water to the ditch from the laundry tub after running a chlorinated wash cycle... over-cautious, but I have the pump and hose anyway. However the septic tank can deal with some bleach.
https://www.clorox.com/dr-laundry/septic-tanks-and-bleach/
It is not good for grass, but some people have a ditch which is not quite grass-- often just weeds. Any chlorine entering the river would become diluted very quickly in the river, presuming the river flows.
Back to your main topic... If you had a 6 inch casing, you might be able to drop a couple 1 inch slow-release pool tablets to the bottom. Then watch your water with a chlorine test sensitive to 0.25 ppm chlorine to make sure that you are getting some chlorine but making sure you don't get excessive chlorine. With 5 inch casing, it seems to me that the tablets are much less likely to make it to the bottom, and they may not make it with 6 inch. Very experimental I think. And certainly up to professional standards.