Add crash-reports with sites like https://mclo.gs/
Update CyclopsCore to this build:
https://www.curseforge.com/minecraft/mc-mods/cyclops-core/files/6199446
Yes, once you have data generation set up, it's a lot easier to add a line of code to your generator class than it is to copy, paste and edit 2-3 JSON files for each new item.
Minecraft 1.21.4 requires a new model definition file for each item, which you don't have. These can be created through Data Generation, specifically the ModelProvider, BlockModelGenerators and ItemModelGenerators classes.