farmfamily,
I know I am chiming in late here, but I wanted to pop-in and mention a few things that may be of help as you ponder your child's situation.

Spelling in the early years is often quite tied to a child's reading, because kiddos at the early stages of spelling are often sounding their spelling words out as they write them. So, in the early years, as your child's reading progresses, the spelling will lag a bit behind that reading progress in coming along too. This is pretty much what you're seeing now, as your child's reading is coming along and the spelling is now progressing a bit more too. That is not to say that in the long haul spelling and reading progress are always linked as that is not necessarily true, as the spelling words get longer and harder.
Another thing to keep in mind is that for kiddos to whom spelling does not come naturally more drill is not really what they need. Regular practice in capturing the correct mental image of a word is the skill that truly needs to be developed in order for the mind to know whether a word that has been written is written correctly. This is the skill that is being developed in Beyond and Bigger. It is also one of the reasons why the other writing the child is doing during that season of learning is kept to copywork or copying from a correctly written model, because we don't want the mind capturing the incorrect image (and having a child inventively spell many words results in the incorrect spelling beginning to "look right" in the mind's eye.) So, to prevent this same thing from happening during spelling lessons be sure to immediately erase any incorrectly spelled word and have the child copy the correct spelling over top of the erased word instead.

Think of spelling time as mental training rather than seeking memorization of specific words.

In that way, every error is an opportunity to swoop in and retrain the mind. Be vigilant as you do the spelling lessons. As soon as an incorrect letter is written in the spelling of a word, erase it away and redirect to the correct image (showing the index card with the correct spelling upon it). Be sure to use a dark colored marker on a white index card too when writing the spelling words (as directed in the guide), which helps the mind capture the image of the word even more clearly. Over time you will see continued progress.
Dictation builds on the foundation of mental picturing that is practiced in the spelling lists in Beyond and Bigger. It is where kiddos actually start to pay more attention to spelling in the context of sentences. It is the moment where they realize spelling is about writing a string of words correctly. It is mental imaging taken to the next level.

This is often where kiddos start doing a bit better in spelling, if they had a hard time in the word lists that they did before beginning dictation. This is because in dictation they are putting to use the mental imaging and beginning proofreading and auditory skills they practiced in Beyond and Bigger and are applying them. Through studied dictation kiddos learn to transfer the skills of capturing a correct mental image of a string of words, auditorily hearing the sentence and repeating it back correctly, writing the words in the correct sequence (including all punctuation and capitalization), and proofreading and correcting their work to make sure the right mental image remains (rather than the wrong one). Over time, these skills transfer to kiddos' proofreading their own written work in other subjects. You can see this is all a part of spelling, but it is a process that takes years to internalize.
This is why I encourage you to keep on going, patiently guiding and diligently correcting. You will see progress as the years pass. Just make sure not to put the focus on word memorization but rather place the focus on the ultimate long-term goal of writing correctly and proofreading in daily work.
My own third little guy struggled with the spelling lists in Beyond and Bigger too. He improved as he headed into dictation, even though he is by no means a natural speller. Now that he is in CTC he is really starting to show some carryover and improvement in his daily written work. He is learning to refer back to his reading material to copy the correct spelling of words within his written narrations. This is another moment where capturing the correct mental image of words (i.e. names and places) and transferring them to paper in this written narrations comes in handy. I share this to encourage you that over time with these methods, even kiddos who struggle with spelling will make gains in the area where it really counts.
Blessings,
Carrie