For anyone else having similar issues, I have since found a bit of a work around to get it loading properly for me. I teleported to a random spot in my world that I haven't touched/developed and then closed the game. I then reloaded it in 1.17, gave it a few minutes to load some chunks, then closed the game again, then reloaded in 1.18. This seems to have fixed it and rebooted whatever runs to generate terrain, biomes and structures. This has left some hard chunk borders, but it's a small price to pay not to lose the world entirely.
For anyone else having similar issues, I have since found a bit of a work around to get it loading properly for me. I teleported to a random spot in my world that I haven't touched/developed and then closed the game. I then reloaded it in 1.17, gave it a few minutes to load some chunks, then closed the game again, then reloaded in 1.18. This seems to have fixed it and rebooted whatever runs to generate terrain, biomes and structures. This has left some hard chunk borders, but it's a small price to pay not to lose the world entirely.