Hi, it would be amazing if there could be custom geodes with the custom blocks created by this mod. Basically identical geodes to the Amethyst ones with the amethyst and budding amethyst blocks replaced by the custom ones, with config options via JSON files for setting the Y-level for said geodes...

This is unfortunately not viable to include in the base mod. With datapacks becoming increasingly powerful in almost every new MC version, the dynamic generation of the data needed to make the blocks work properly (models, loot tables, etc.) becomes more and more complex. Generating the data that's necessary to create the geodes is fairly involved and not feasible to do dynamically.
The only thing I could offer is a template datapack that adds a geode for one of the builtin budding block types. I'll keep this issue open until I've decided how to proceed with this option.

There's no way to duplicate Amethyst geodes and simply change the blocks to the custom ones? I understand this is a complex issue, I would only need it for 1.20.1 Forge. If not I understand and thank you for your reply!

Worldgen is controlled fully by datapacks. To make geodes work, several JSON files in a datapack are needed and those JSON files are fairly involved, it's not as simple as just adding or changing a single line somewhere. If I add an example for this, then it needs to first work in 1.21 once the port to that version is completed and only after that will this be backported.