But baptism is not a secondary, unclear or ambiguous belief ... It is, as you have agreed with me, a foundational Christian belief
Yes, I do and indeed must agree, for Hebrews 6.1-2 puts it there, and of course the command to repent, believe and be baptised is frequent in Holy Writ. What we are discussing is not whether baptism is optional - we all agree that it is not. Rather, we are debating the question of when it should be applied. My friends who were 'baptised' as infants sincerely regard themselves as baptised believers; they are not consciously disobeying the Lord's command, and indeed in the years 1963-1968 I received much blessing during my time in Methodist and Anglican churches. Also, I suspect you believe that the person baptising must be a priest within apostolic succession.
In addition, discussion (not here, I think) sometimes wanders into whether baptism should be by immersion, as we practise, by affusion (pouring), or by sprinkling. We all agree that it must be in water. Here, we have also entered into the matter of the inner meaning, symbolism, efficacy, grace (however one sees it) of baptism. It is my suspicion that if a person genuinely believes he is a baptised Christian, then God accepts that sincere belief and intention, even though some of us have obviously misunderstood some points - not that that means we can assume liberty not to do our utmost to obey what we do believe is the command.
But we all agree that baptism is not an optional extra for specially religious people.
This opens a different question: what do we make of people like the Salvation Army, or the early Quakers, with no baptism? I think they are disobedient to the Lord's command, but I find it hard to view them as 'unsaved' (as we Vangies say): reading George Fox or William Booth shows too much of Christ in their thought and life to make that a credible conclusion, I think.