I have processed many hundreds of thousands of raster images in a fast batch. I can tell you that nothing matches Rasterex in raw power and speed. My memory doesn't allow me to tell you the exact max specs for PNG, but I am sure what you have named is ok, because what I dealt with was typically 30x42 (@400dpi) so 1200x16800 and frequently I would dump an oversized aircraft 1:1 page in, which could be 42"x12' or much longer at that same 400 dpi, and it would not even hick up.
One word of warning: Pay attention what OS you are using. In my experience nothing could beat Windows2000. Windows XP had the same limitation but bigger overhead and was noticeably slower. Forget about Windows ME, 8, 10, 11. Windows 7 would churn, but just so, throwing limitation errors.
This is because Microsoft had realized that they have sold to the people a commercial-power OS, capable of running anything, and yet the people mostly had no idea, so it was possible to take it back version by version, which was the true reason for the updates and the upgrades, by now everyone has just a weak OS, not capable of anything at all, and essentially just a platform on which to run the software they sell. So, if you think that limitations you see come from the software only, sometimes it is being triggered by the OS API calls and the regression is clearly observable.
Rasterex used to be sold for thousands of dollars to the Reprographics companies on CDS, with a hardware key for a parallel port. I expect some companies to still have it dusting in a software cabinet somewhere in the pile of windows 98 circa old refuse they own. Today, no one working there would be able to know what it is, and they would be happy to give it away just for asking.
I do not know if it has been produced or not. Last version I saw was a reversed engineered v2000 (I think) with all license code stripped. x32 and wont run on above XP.
Give it a try if you have a headache.
What you have is a fiche strip 8.75"x173' if at @400 when digitized. It could be at 600. I don't think fiche was ever mass scanned to digital at more than 600 dpi.
Consecutively, your main issue is the length. You need to cut them up length wize, if everything else falls.
The only tool that has no length limitation is Reprodesk (Server, not free viewer), again for around 2000-2002. This tool used to come with massive and complicated licensing structure from Autodesk, and IIRC before 2002 was physically bound to an internal processor of a TDS digital printer produced by OCE. They cut if free when the processing power (then with Windows2000 was able to match that of the 2 tonne TDS printer circuit board).
There were some cracked versions too. Again start by asking old IT guys at old Reprographics shops for what they might have stored out of no desire to clean up.
Everything above failing, what you need to do is to use freelance coders who understand raster format images to make a simple utility just for you to arbitrarily cut the PNG into fragments say 42 inches long (the max print width of most digital printers), by opening as a file and inserting end of image tags and saving as pieces. This is totally possible if the guy actually knows what he is doing. The limitations will come up only when attempting to open as image for viewing. This is why you may have to cut arbitrarily.
Once you have pieces, batch process (saturation, etc, filtering etc, renaming etc) using anything or even those same Rasterex and Reprodesk, then line up and merge if necessary using either of them. Rasterex will process on the fly all the editing commands, and even some (automated) merging with batch scripting. Reprodesk really speeds up manual alignment, manual merging, and visual control before a RIP is committed. Rasterex again, can finally take them at once and batch auto de-skew determining the angle each time on the fly in milliseconds.
I am assuming you have thousands of these. This is what I would do. Good luck.