I think the kāya here is breath what is known thanks to the breath and is belonging to the feeling khandha not rūpa,https://suttacentral.net/sn12.31/en/sujato?layout=sidebyside&reference=none¬es=asterisk&highlight=false&script=latin wrote:“Sir, one truly sees with right wisdom that this has come to be.
“Bhūtamidanti, bhante, yathābhūtaṁ sammappaññāya passati.
Seeing this, one is practicing for disillusionment, dispassion, and cessation regarding what has come to be.
Bhūtamidanti yathābhūtaṁ sammappaññāya disvā bhūtassa nibbidāya virāgāya nirodhāya paṭipanno hoti.
One truly sees with right wisdom that it originated with that as fuel.
Tadāhārasambhavanti yathābhūtaṁ sammappaññāya passati.
according to Visuddhimagga breath is known by the contact.https://suttacentral.net/mn118/en/sujato?layout=sidebyside&reference=none¬es=asterisk&highlight=false&script=latin wrote:When breathing in heavily they know: ‘I’m breathing in heavily.’ When breathing out heavily they know: ‘I’m breathing out heavily.’
Dīghaṁ vā assasanto ‘dīghaṁ assasāmī’ti pajānāti, dīghaṁ vā passasanto ‘dīghaṁ passasāmī’ti pajānāti;
..
They practice breathing in experiencing the whole body. They practice breathing out experiencing the whole body.
‘sabbakāyapaṭisaṁvedī assasissāmī’ti sikkhati, ‘sabbakāyapaṭisaṁvedī passasissāmī’ti sikkhati;
kayasankhara is a feeling what is felt by the contact, i think its the internal breath
https://suttacentral.net/mn118/en/sujato?layout=sidebyside&reference=none¬es=asterisk&highlight=false&script=latin wrote:They practice breathing in stilling the body’s motion. They practice breathing out stilling the body’s motion.
‘passambhayaṁ kāyasaṅkhāraṁ assasissāmī’ti sikkhati, ‘passambhayaṁ kāyasaṅkhāraṁ passasissāmī’ti sikkhati.