I really want to do this to support a minority language (Kouri-vini) that currently has zero input support but an existing digitized dictionary. I also have zero coding experience but I don't care if this takes me 10 years, I'll find a way to do it.
Custom Keyboards are commonly made with the Ukelele app. For input to work right you really need a Unicode font for the language. Spellcheck, etc is not connected to either the keyboard or the font, but requires .dic and .aff dictionary files. Info on how these can be made is available at
Whether a custom spellcheck dictionary will also do autocorrect I don't know.
For Kouri-Vini you don't need a special keyboard or font, just the dictionary.