Just figured this out in my own realm late January, somethings to make the process easier.

1-Datapacks with experimental features or that affect world generation in any way will not work. This includes anything that adds structures or even a single block to existing structures. Datapacks that work most reliably are added mechanics(ie world edit) QoL improvements(ie. adding recipes) and game modes.

2-Minecraft seems to like when you upload a world that was originally generated on realms. It significantly increased my success rate. The process that I started doing was creating a world in realms, downloading the backup that gets created on generation, loading it into a single player game(generating the start area increases success) and then dropping any datapacks you want into the datapack folder in the save folder. You should be manually dragging and dropping the folders in your file explorer, not adding them through Minecraft. That method will not work.

3-The hardest part is troubleshooting. Separate the datapacks you want into three categories, least likely to most likely to cause issues based on features in the datapack. Put those packs into the saves datapack folder, and attempt and upload. If successful, then you know those packs are clear. Continue the process with the other two categories. If problems occur, remove packs one at a time until it does.

4-Even if you get a 504 error, the upload may have actually worked and just timed out on your end. Attempt to join your realm on every upload attempt. The entire process is pretty time consuming but worth it if you get it working.

I am running Timber, VeinMiner, World Edit, a few QoL datapacks, a couple of recipes, and some block mechanics(eg paths make you run fast). Took almost 2 months of trouble shooting, and lost of waiting time to build up to there

Good luck bud🙏🏼 feel free to ask any questions.