Is there some kind of guide as to what choices in the prologue make you end up as? (And whether it's possible to end up as someone that can impregnate others and get pregnant?)
Choices in the prologue generally steer you towards one of two tracks, either making the MC lean more domineering or more seductive. Knowing that, most choices with two options will be reasonably obvious. Similar goes for body shape: When the MC is asked whether they prefer petite or large proportions, or their thoughts on hair length, those choices will inform the transformation.
(The MC can end up with a penis or a vagina but not both, so impregnation
or pregnancy are possible, but not both.)
Might be a silly question but can you rather go the opposite way & instead just become submissive?
Asking to make sure: Can you become a full (non-futa) woman or is this another one of those games with just sissification / trans?
I don't believe there's any submissive track in this, no. The MC can become forcefully domineering or seductively manipulating, but this is one of the few tf games that don't involve the MC becoming submissive or sex-obsessed or otherwise losing their agency.
And yeah, as Epsilon said above, the full woman-with-vagina transformation is the most likely outcome, unless the MC makes specific choices in the prologue to preserve their current downstairs plumbing.