Well, many have described Millard as manipulative, and IMO I think he would have thought that the idea of representing himself would be of some advantage to him, besides causing trial delays. As I said, perhaps not clearly again, it gives him the opportunity to present his 'story' written to fit the evidence to a lawyer he planned from the beginning to retain. Just MOO though.
I think if he had wanted to put on a show, he would have taken the stand at the Bosma trial, which would have been hopeless at that point and to gain any credibility, he would have had to have shown some form of remorse or show of sympathy toward the Bosma family, but he probably could have done a lot of damage to Smich in spite of the throttling he would have been served by TD. JMO though.
ETA: IDK why he ultimately chose not to testify, if it may have been that he was loathe to show remorse or sympathy, or that he was so intimidated by Dungee, or both among other things