Welcome to the Forum, Glad you found us in our corner of the web.
I can see the issue you are having and yes I agree it’s under extrusion. I generally break this down into one of 5 sources, Filament, Slicer settings, Extruder Mechanics, Hot end issues (generally partial clogging. Worst case scenario it could be the main board or specifically the driver.
Given the clues you have given us in the description, I would tend to eliminate filament immediately, but If you want to exclude it permanently. I am assuming this is happening with multiple rolls of Filament as you mentioned it’s getting worse over time. If it does happen with multiple filament rolls we can safely rule that out for now.
It seems to work for a little while and appears to be quite good, then goes through a period of under extrusion and then seems to recover again for a period of time. So with this being observed, I would tend to look past slicer settings, Most commonly when someone creates a new printer the default cura Filament size is 2.85 and when you start printing with your new profile it under extrudes all the time. It’s not good than bad, then the good type of scenario. So I would at least set aside the slicer option although a look through your settings to ensure something is not changed accidentally would not hurt.
I think you are looking at some type of mechanical issue, So now using the process of elimination we have to decide extruder or Hot end. Let’s get the hot end out of the way first with a quick rebuild, Put a new nozzle in it because I have seen bad ones right out of the box or if you happen to touch a hot brass nozzle onto the bed it can deform the orface pretty quickly. While it’s apart check the throat and heartbreak for being bent, bad threads or debris building up in the inside of the throat. Ensure your Bowden fitting is solid and not slipping, ensure you reinstall the blue clip on both ends of the Bowden tube. Always remember to heat-tighten the nozzle after a teardown and reinstall the silicone sock.
Now for the extruder, I believe the issue is more than likely here as your 100mm extrusion tests are somewhat unpredictable, from 50mm to 86mm to 86mm is not a good sign. Creality printers are pretty standard and should be somewhere on an e-step of between 90 and 98. Anything outside of that its mechanical. I know you have checked the lever arm, but you may want to check the bearing on that arm as well, Make sure its not seizing up, Check the brass drive gear for excessive wear and that the center of the drive gear is still lined up with the center of the filament path hole in the extruder. Ensure your set screws on the brass gear are snugged properly, Loosen both, Tighten the one on the flat surface of the extruder motor first and after that tighten up the second one. Check the temp on your Stepper motor, and ensure it’s not getting too hot, during normal operation it should get warm but never HOT. Inspect the wiring for broken cable, Loose pins or connections, perhaps pinched in a corner of the frame.
I know I have thrown a lot of information at you on this one but it can act as a bit of a guide for anyone else having similar issues.
Let me know how you make out and if need be we will continue troubleshooting with you.