I would like to move my survival world from my Mac (Big Sur, Minecraft Java Edition 1.16.3) to my iPhone (iOS 14.3, Minecraft Bedrock Edition v1.16.201) but the only free software I could find was MCC Toolchest, (which only runs on Windows) and Anvil to LevelDB (Which I couldn't find a valid download link for). I'm wondering if there is an easy way to manually convert the world.
There's a very good Minecraft converter for both Editions Java and Bedrock! This app was made by Hive games, you can convert Java Edition worlds to Pocket Edition worlds very easily with this tool!
Unfortunately, I don't know of any good way either. Perhaps you could use some other software that lets you run windows apps. I tried it with Crossover, but didn't make much headway there because of some missing dependencies.
As for seed conversion, that's kind of a myth. The nerdy answer as to why it doesn't work is that bedrock uses 32-bit and not 64-bit seeds and the two games also use completely different random number generators. Those RNGs are used for world generation, so even if you input the same seed, the game will generate entirely different worlds. That also means that world converters will only successfully convert loaded chunks; unloaded chunks will be generated with bedrock RNG, so you'll only have the "same seed" effect for parts of the world that have been loaded at some point (places the player(s) have seen).
Anyway, hope this is informative! If I find a better way that doesn't require an insane amount of coding, I'll try to let you know. :)