But the trinity is a later Greek invention that has nothing to do with the religion of Jesus. It's a completely made up concept which was added centuries later.
One of the whole points of Islam was to recognise this, and seek to return to the original monotheism. Why are there three gods in Christianity? That's what the trinity effectively is. It's a denial of the unity of God.
In Islam, God's one-ness is a central aspect of the faith. Tawhid is the term used to describe belief in the unity of God. As you correctly said, Islam was not a "new" religion. It was a continuation of the original Abrahamic faith. If anyone is starting a new religion, it's Christianity.
If the trinity is made up and we believe an historical Jesus existed, then it was the very people who write the New testament that made it up, not people centuries later.
Islam is much more than simply some kind of return to this apparent original monotheism of Jesus, if one of the 2 religion is true the other becomes hogwash as the 2 contradict one another in many different ways, other than just the Trinity.
Islam was practically pretty much a new religion even if it didn't saw itself as a new one, I'm not strictly sure where the evidence for strict monotheism in early Christianity comes from, it's not like we have much more than the religious texts left by early Christians, at this point believing that early Christianity was monotheistic is not a position based on history but based on believing a specific narrative, regardless of us treating the religions, stories and religious texts as pure fiction or not.