Well, people’s beliefs do change and evolve during their lifetimes, and I’m pretty sure this is the case for Constantine, too. How much of a Christian was he at the time of the Milvian Bridge, in 312? Well, he had his alleged vision followed by a dream, but maybe that was retconned later, either by his biographers or by an older and more devout Constantine. There is famously no overtly Christian symbolism on the Arch of Constantine, erected in 315 to commemorate the victory. And, yes, that is a bit weird, as everyone agrees. Although, the inscription on it does attribute the victory to a “divinity” (unspecified). The Edict of Milan, from 313, legalizes Christianity, proclaims religious tolerance, and instructs that confiscated property should be returned to Christians. However, it does not outlaw paganism or make Christianity the official Roman religion. (All that would come later, but not until Theodosius in 380, well after Constantine’s time.) The edict is actually rather wishy-washy in its language concerning God, speaking again of “the divine” (unspecified). Constantine does make use of the labarum standard, with the Chi Ro symbol, from at least as early as 317, when it’s depicted on coins. However, it’s not entirely clear that Constantine would have considered the Chi Ro to have been an exclusively Christian symbol, as opposed to one way among several he could get the attention and support of a “divinity”. Until 324, he was also very much into the cult of Sol Invictus, as noted earlier. So, at this stage? Vague monotheism?
Constantine would later, of course, heavily promote Christianity, construct basilicas, and take a personal interest in church affairs. I think he was schooled pretty heavily in Christianity at around the the time of the Council of Nicea, and he ditched Sol Invictus at that time. Basically, the older Constantine was probably more Christian, more knowledgeable about Christianity, and more exclusively Christian, than the younger version. And is there really anything wrong or all that unusual about that?
Yes, he was only baptized right before his death. It may indeed have been an attempt to play chicken with God, and have all the sins of his lifetime (of which there were many, BTW, so he certainly had something to play for) erased at the last moment, allowing him to enter the afterlife with a blank slate. But then again, this is a guy who was progressively moving in that direction over time. Maybe he just wasn’t ready to take that final plunge until the last moment.