For anyone struggling with this, I have a solution. After hours of trying to work this out, I found a way that worked for my 477dw:

Download the flash file which is named like: maverickdw_dist_pp1_001.1908C_nonassert_appsigned_lbi_rootfs_secure_signed.ful2 where 1908 seems to denote the year and month like august 2019.

The USB didn't work for me. It was stuck at verifying. I entered a special blue menu by turning off the machine and when turning it on, holding it until the screen was black and the controls lit up.
Touching house, back, house house in quick succession opens the menu in which you can go menu -> reflash. This didn't work for me.

Now what I did was:
1. Find out the printer IP adress (can be found in the printer when clicking the antenna icon
2. Put that into the adress bar of your browser and open it. These are the settings of your printer.
3. Go to settings -> Security -> Aministrator settings and change the printer updates and enable "All firmware updates" and disbale "Only Web-Downloaded Firmware Updates"

This might actually enable you to use the USB stick method, but I am not sure.

4. Activate "LPR Port Monitor" on your Computer.
- Go to "Activate Windows-Features" or similar
- search for "Print and Document services" or similar
- Check "LPR-Connection Monitor" or similar.

5. Send the flash file from your pcs command interface
- open cmd as administrator on your pc
- navigate to the directory where your flash file is like "cd C:\foldername"
- enter this command: lpr -S -P raw
Where is the IP from the previous steps and is the name of the flash file.

Use your favorite AI to help with this if you are confused. But what it does is: it sends the firmware to the printer as a print job. You will hear a pling from the machine and it will be unresponsive for a while (5 minutes) after which it restarts and is flashed.

Lastly you need to disable the auto updates!
First of all, you unplug your wifi just to be safe.
You go into settings -> Printer Maintenance -> Update the Printer -> Printer Update Options or similar. And now disable the updates entirely. The popup will be confusing and yes will mean no and vice versa. You DO NOT WANT UPDATES.

You are almost done. Now just to be extra safe: disable any HP software from your computer so it doesn't update the printer. And you are done!

I am so furious that I was barred from using the already extremely expensive cartridges I bought, that I am making this very detailed post out of spite for HP. It is not as hard as it sounds and saved my hundreds of dollars. The thing that was missing for me, was that i had the "Printer Updates" disabled in the menu which you can access via your browser.

If the file ever gets taken down from this site, you will find similar files on other sites and in the descriptions of youtube videos around this topic. Good luck