I've tried to download the latest backup from my realms world numerous times now, and they always fail. I've read about a max world size, but as Mojang didnt give us a chance to know, what our backup/world size currently is, i don't know how I would know if that was the issue. I REALLY do hope, that they wouldnt let me know if the backup was undownloadable and not just let me download the first 5gb, and then just be like "oh yeah, btw this is over 5gb, so you cant continue".

The download fails on different percentages each time.

Some times it fails on 2-3%, sometimes on 29%, sometimes on 79%.
Some times it says "Download failed" instead of "Downloading" above the speed and progress, even though both the percentage done, and download speed are still changing. Then after a while, as sure as day, after a while it will say Download failed with red text underneath progress instead, and now the progress is stopped.

I've tried while launching the game from the official minecraft launcher, as well as game instances lauched through MultiMC.

I've tried on multiple machines, all with plenty of space to spare, ethernet cable (500/500MBit internet connection) attached, and different OS's (Win10 and Arch Linux) and yet no luck.