It might be helpful to know that the influence of the Upanisads on EBT is negligible. I do think that the old Up. are pre-Buddha but their knowledge starts to influence the texts only in the later suttas - this is probably due to geography. So Vedic texts have more chances to be influences on the Buddha’s thoughtworld when they’re older - Satapatha and Jaiminiya Brahmana, Rgveda and Atharvaveda.
I recommend Lauren Bausch’s excellent PhD thesis “Kosalan Philosophy in the Kāṇva Śatapatha Brāhmaṇa and the Suttanipāta”
The older influences are obvious in the Buddhist pantheon, where of course deites like the 33 Gods and Sakka/Indra are of ancient Vedic origin.
Other important Vedic influences are the ideas of religious giving and brahmacariya. James Egge wrote “Religious Giving and the Invention of Karma in Theravada Buddhism”. About Brahmacarya there is some literature (e.g. by Mieko Kajihara), but it’s not related to Buddhism. I write about it in my thesis, but I can’t share it yet until the process is finished.
In contrast to the Vedic literature (which is just too much) I recommend reading Jain literature in the source. Next to the texts you mentioned (mostly Acaranga 1 !) I would surely read the Isibhasiyaim, which is ancient but not included in the canon (because it’s para-sectarian).