My point is that there seems to be no sane way for an adult person to convert to a theism.
A theism like that of your religion is a closed, self-referential system, which is bound to verify itself, of course. But an adult outsider can't enter such a closed system, other than by a leap of faith.
I agree, when we are talking about the 'just have faith in the Bible/Quran/Gita' varieties of fideist theism. That's just circular and hence epistemologically empty in my opinion: Simply will yourself to believe that X is true, and you will believe that X is true. Unfortunately, that path would seem to work for any X, even psychotic delusions. I fear it as the path to madness.
But it it's conceivably possible to approach theism in the manner that the Kalama sutta suggests. A theist might tell a non-theist that 'here's a method, a practice, that leads those who follow it, even individuals who aren't already convinced that the path works or that God exists, to knowledge of God (in the strong personal experience sense, as opposed to the weak inferential sense).
I've always been kind of attracted to the theistic mystical/contemplative traditions for that reason. (Not interested enough to follow them though.) Unfortunately, many of these theistic paths do seem to presuppose a preexisting belief, and even make one's success in following the path a function of one's faith.
Typically, for the greater part of human history, people have been born into such systems, taking on the religion of their parents from birth on. Which is why, for the greater part of human history, there didn't exist problems related to conversion as they do today, when more and more people are born outside of religion and somehow have to find a way to enter it as adults.
Or when they fall out of their birth-religion, and then thrash around crazily looking for a substitute (whether another religion or some ostensibly secular belief system like Marxism). I think that more and more people are falling out of their birth-religions, because there are viable alternatives available in their own communities today that didn't exist in the past, and because religion today is far more perfunctory for most secular-but-nominally-religious people than it might have been for more devout people in ages past.
But as soon as a preacher preaching such a closed system to outsiders resorts to supposed "reasons for believing in God" and "reasons for joining a particular theism" - this is when things become irrational.
Maybe not entirely irrational, but ultimately unconvincing and unsuccessful. The Buddha seems to have noted it in the Kalama sutta. (Actually, I'm unsure whether the failure of the theistic arguments is due to logical defects, or to the simple doubtfulness of the arguments' axiomatic assumptions.)
You are ascribing to the Buddhism of the Pali Canon an outlook that it doesn't claim to have.
It isn't a matter of subjecting one's self to the Buddha, or to some contemporary who claims to represent the Buddha. That sounds more like guru-worship to me. (Actually, I think that we sometimes do see that in some tantric-influenced varieties of Buddhism and perhaps even in Zen. But as you say, not in the Buddhism of the Pali canon.)
Having said that, there is some basic requirement for faith, in the minimal sense that a Buddhist needs to have enough faith in Buddhism to think that it's worthwhile to engage in Buddhist practice. But that doesn't seem to be anything remotely like the kind of circular supposedly self-verifying faith that some theists exalt.
IOW, you are treating the Buddhism of the Pali Canon as a religion, as a set of doctrines and authoritative instances. And while the later Buddhist traditions certainly function they way religions typically function, the Buddhism of the Pali Canon is not like that.
Or if it is, it's a set of authoritative instances that define Theravada Buddhist practice, especially for monastics. It isn't really a set of propositions whose truth Buddhists are expected to believe on faith. The truth of the propositions is something that's said to become apparent to practitioners in the course of their own direct experience.