I've developed film fifty years after exposure, and several times twenty years or more. "Found film" is a somewhat recognized sub-hobby of thrift store camera shopping.
That said, sooner is generally better. Is it possible you could buy a bottle or two of one of the currently available monobaths (developer and fixer all in one bottle), a ...
35mm film negatives are the ultimate archival medium.
Digital technology shelf life is limited. Good luck
with hard drive issues, etc.
35mm film cameras were built to last, digital formats
break down much quicker, expensive to repair than 35mm.
Photography was made to be captured on 35mm or medium
format film. Digital tech. a branch of photography.
It does look somewhat like the kind of marks you can get when the film is not perfectly wound on the reel. Often you would get completely undeveloped bands running across the width of the film, but you can also get areas that developed incompletely. Any kink or overlap on the reel causes this.
But there could certainly be other causes, from improper ...
I think the artifacts are too vivid to be residue from the rinse agent. If re-washing and re-rinse agent did the trick then OK. If not - re-fix twice normal fix time and wash and rinse. I think the film is properly fixed. If I am right, then this will do the trick.
Those marks run the long way (assuming it's 35mm, which would fit with your 300 ml developer volume). That pretty much means they got there when you hung the film to dry, which means they're most likely due to mismeasuring your wetting agent for the final rinse. Too much wetting agent can leave a residue that dries on the film, rather than running off ...
A lot might depend upon how the negatives are currently cataloged and stored. Are they in translucent sleeves that allow low-quality scanning without removing them from the sleeves? Or are they in paper sleeves/envelopes? Is there any type of systematic cataloging naming or numbering scheme already in place that would make retrieving negatives selected for ...
Not a task I've ever needed to do, but why not kill two birds with one stone & just outsource the job to a decent photo lab?
They could provide full-scale images & thumbs [contact sheet quality] all in one pass.
You could then reference one to the other by filename & keep/discard in pairs.
Snapping each one with a phone just sounds like masochism ...
What you are seeing is called "Reciprocity Failure" of color negative film from under-exposure.
Color film works by having multiple layers of chemicals (emulsion) that are sensitive to different colors (light wavelengths). When there is very little light to collect, you see color shifts because some emulsions are more sensitive to light than others,...