A spelling course could be designed in a similar way to the grammar course (don't worry about grade level, just separate based on large differences, make videos and practice examples, have unit tests).  I understand that there would be more (English has way too many rules, exceptions to rules, and words that outright defy explanation), and making it clear whether it is US or UK based spelling rules would be crucial,  HOWEVER, I think this is an area that students could do self-paced, and it could also be assigned along with grammar and the ELA courses by ELA and homeschool teachers.

Personally, I'd like to assign a voice that says the word, then the exercises could be a mix of blanks for students to write the words themselves and multiple choice.