If the Doctor is, in fact, a space jellyfish and an energy-being, then - based on the limited counterexample of the one other qurunx we've met - the Doctor's lifestyle of continually converting their energy into matter and taking organic forms, seems to be kind of freak shit for their species? The other qurunx was just projecting a hologram of a child. The Doctor has made thenself physiologically indistinguishable from a regular gallifreyan/shobogan, aside from that lingering Jellyfish Energy that lets them regenerate
Had the "original" child already taken on a shobogan form before Tecteun found them? They don't seem to have "regenerated" in response to her (though for all we know, regeneration/shapeshifting might have worked completely differently before the "rules" of it were drilled into them), so they didn't necessarily become a flesh-and-blood shobogan in order to appeal to her. Could they have been shapeshifted into a different humanoid species? Were they still more projection than flesh at that point?
But we can assume they'd become a Real Biological Shobogan by the time Tecteun started doing a battery of medical exams after them (which is why it took so long for her to isolate the Jellyfish Genes)
Which means the point when they become a "real" flesh-and-blood gallifreyan, two hearts and all...may have been their first regeneration after being adopted. When the Baby Master (accidentally?) pushed them off that cliff
Qurunx's take a form that they think the person looking at them will want to protect
The Doctor was killed by their friend, and their dying thought was "I want to become something you can love." And that desire was so powerful, it shaped them into a Time Lord from that point onward
The Master has a breakdown about the fact that he was remade in the Doctor's image. Hybridized with something unknown. Whatever he used to be - his "true" species, his first body, his original self - has been burned away, ship of theseus'ed into oblivion
But the Doctor also remade themself in the Master's image. So throughly that their "original" form is just as lost to them
I wonder who I'd be, without you