You will still be able to download anything you've previously purchased https://en-americas-support.nintendo.com/app/answers/detail/a_id/57847/%7E/wii-u-%26-nintendo-3ds-eshop-discontinuation#s1q4 "for the foreseeable future":
How long will it still be possible to redownload previously purchased content in Nintendo eShop for Wii U or the Nintendo 3DS family of systems? How long will those systems still support online play?
For the foreseeable future, it will still be possible to redownload games and DLC, receive software updates and enjoy online play on Wii U and the Nintendo 3DS family of systems.
Thus, there isn't any immediate need to backup your Wii U. I did however find https://cemu.cfw.guide/dumping-games.html#requirements , which requires a FAT32 USB or SD Card. It uses your Wii's browser to run an application for you to download your data to your computer. It sounds like a lengthy process, but it's something to try at least. As always, do this at your own risk!
You'll need to follow these steps:
Turn on your Wii U console
If prompted, do not format any USB devices through the Wii U
Ensure that the title you want to dump has the latest update and any relevant DLC installed
Some games, such as Breath of the Wild, require an update to function correctly in Cemu
If using a disk, ensure it is inserted before launching dumpling
Insert your USB storage device or SD card
Launch the internet browser and open dumplingapp.com
Tap Launch dumpling
If your console freezes for more than 10 seconds, hold down the Power button for 4 seconds and reboot
Once rebooted, reset the browser's save data and try again
Using the dumpling app
Select Dump digital games or Dump a game disc depending on your scenario
For Digital, select your games then press (START)
For Disc, continue below
Verify that the dump's destination is correct (SD or USB)
Ensure your desired Wii U Account is set
Select Start to begin dumping
This can take a very long time, we encourage you to dump one game at first.
Once finished, dump your next game at Step 1 or turn off your Wii U and put your SD or USB that you dumped to into your computer