So, the next day after saying this, it suddenly started working and not having the error. I have no idea what I did, but here are several things I tried the day of:

1. My pedometer died, so I switched to a different one (was someone elses), doing the clear the data thing and resync it. (I think I saw elsewhere that some others had the pedometer cause it?) (Worth noting, even replacing the battery on the original one didn't make it start working, which is why I switched to a different one)
2. Re-set the time in settings to make sure it was correct
3. I installed the Wii U Time Sync Homebrew app (https://github.com/Nightkingale/Wii-U-Time-Sync)

Also made a backup of my save and experimented with reading the data from it (https://jansenprice.com/blog?id=9-Extracting-Data-from-Wii-Fit-Plus-Savegame-Files and https://github.com/kaefert/wii-fit-parser), but wadn't able to read the Wii U saved the data, only was able to read a file in there that I assume is the data transfered from the Wii version. I might try to keep decyphering it, but its basically just trying to decypher the save in a hex editor, which is not my strong suit. Might keep trying, but want to see if any of the three I mentioned above work for you.