So I changed the hotend (twice even)
Including the nozzle? That was my first guess.
After reading everything I'm thinking "feeder". The feeder should be able to push about 10 pounds (what country are you in - please update your location settings in your profile).
when sliding filament through bowden by hand with everything cold and off - how much of that 10 pounds budget does it use? Probably not much.
Test your feeder. Take a 2 foot length of filament and put it in the feeder, attach it to a bucket sitting on a scale on the floor. Use bungee cords and a vice-grip to connect the filament to the bucket. Start turning the feeder by hand. How much does the weight on the scale change? You should be able to do around 5kg or 10 pounds. if you can only do 2 pounds then we know the problem is the feeder.
Here is a photo explaining how to do it - same procedure for UMO or UM2 - same approximate strength in the feeder for UMO or UM2:
Do you have the same problem when using the second extruder (eg; use the second extruder as your 'main' nozzle?)
This could help with narrowing down where the problem is.
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