Just to clarify a couple of issues.
As you hinted at, Williams is not a student of early Buddhism, and I would not rely on his work. The whole thing of rupakaya vs. dhammakaya is not really an issue in the EBTs. Sure, you can infer a few things from isolated cases, but it’s hardly a significant theme.
In the Pali EBTs, dhammakāya occurs only once, in DN 27#9. Check the whole sutta for context, but let me translate the relevant paragraph for you. I haven’t reached this sutta yet, so this is just a rough job.
Tumhe khvattha, vāseṭṭha, nānājaccā nānānāmā nānāgottā nānākulā agārasmā anagāriyaṃ pabbajitā. ‘Ke tumhe’ti—puṭṭhā samānā ‘samaṇā sakyaputtiyāmhā’ti— paṭijānātha. Yassa kho panassa, vāseṭṭha, tathāgate saddhā niviṭṭhā mūlajātā patiṭṭhitā daḷhā asaṃhāriyā samaṇena vā brāhmaṇena vā devena vā mārena vā brahmunā vā kenaci vā lokasmiṃ, tassetaṃ kallaṃ vacanāya: ‘bhagavatomhi putto oraso mukhato jāto dhammajo dhammanimmito dhammadāyādo’ti. Taṃ kissa hetu? Tathāgatassa hetaṃ, vāseṭṭha, adhivacanaṃ ‘dhammakāyo’ itipi, ‘brahmakāyo’ itipi, ‘dhammabhūto’ itipi, ‘brahmabhūto’ itipi.
Vasettha, you are all of different lineages, names, clans, and families, yet you have gone forth from the lay life into homelessness. If you are asked “Who are you?” you should explain that you are ascetics who follow the Sakyan. When someone’s faith is settled, rooted, and planted in the Realized One, so strong that it can’t be shifted by any ascetic or brahmin or god or Māra or Brahmā or by anyone in the world, it is appropriate for them to say: “I am the Buddha’s true-born son, born from his mouth, born of the teaching, created by the teaching, heir to the teaching." Why is that? Because these are all terms for the Realized One: “the body of teaching” and also “the body of holiness” and also “become the teaching” and also “become holy”.
In such contexts the word kāya lies close in sense to the English “corpus”.
The terms dhammabhūta and brahmabhūta occur elsewhere in a couple of stock passages, eg AN 10.115:
For he is the Buddha, who knows and sees. He is vision, he is knowledge, he is the truth, he is holiness. He is the teacher, the proclaimer, the elucidator of meaning, the bestower of the deathless, the lord of truth, the Realized One.
Brahmabhuta also occurs in such passages as MN 94:
The person who doesn’t mortify either themselves or others—living without wishes, extinguished, cooled, experiencing bliss, having become holy in themselves—does not torment themselves or others, both of whom want to be happy and recoil from pain.
The term brahmakāya occurs frequently, but in a quite different sense: the company of deities of Brahma. I am not aware of it occurring elsewhere in this sense.