I had similar problems updating my datapack to 1.17, but I got around them. First to update the datapack, you need to completely rework it to fit the new 1.17 standards. If you wand to see a working example you can look at my datapack here:
https://www.planetminecraft.com/data-pack/apocalypse-overwrite-2-structures-zombies-and-loot
You can also try the Minecraft Wiki, but last I checked the people who write it still haven't finished updating the world gen pages to 1.17.
Reading the logs takes some practice, but it is possible. Whenever it says "No key [whatever]" you need to go into the json file and add that file. For example, add "height":256 into your dimension_type files (judging by the error pasted by HoboMaggot).
Once you get the pack updated, it still might not work, but it can be fixed. Looking through your world save folder, the folder named "region" contains all the overworld chunks, so you can create a new world with the datapack, then just copy and paste the region file. Similarly, the dimensions folder contains all the other dimension chunks, the player data folder contains your inventory, and the entities folder contains mobs. So by copying and pasting folders from the old world into the new one, you can often recover it.
I know it's a lot of work, but you can still save your world. Best of luck!
