Getting M to write anything is a mild form of torture for the both of us, so we do a lot of alternative spelling words practice. He seems to be retaining the words well enough, and I'm not all that keen to force the issue.
This is his favorite: it's a game called Upwords, but we use it just to stack one spelling word on top of the next. So he had rug, rub, tub and then nut, and bruise, cruise and then (we moved the first stack out of the way to make) juice.
I'm hoping it at least helps reinforce how the words are related? And even if not, he'll be really good at those word ladder puzzles when he's older.