Who is the founder of Hindu Religion:
Normally a philosopher/ prophet comes up with a system revolutionary or at least that did not exist before for that set o followers. Accepting the new system disciples and other people start following the religious system and for the new religion that person is the founder.
In case of Hinduism
The system existed from time unmemorable.
Hindu scriptures talk about things that happened at periods of time, whose specified count is much much higher than the period well known to history scholars of current time. (yuga, manvantara, kalpa etc.)
Even the scientific excavations show the existence of well-developed worship, at very early periods. (For example excavations in ancient towns of harappa and Mohenjo daro reveal the ancient worship of pashupati, shiva linga – the Hindu God Shiva.)
There were, of course, very many philosophers who came up with new systems. However given the wide spectrum of Hinduism, its accommodativeness towards new ideas and philosophies all the new changes made it a healthy religion that does not get stagnant.
People used their discretion to find the suitable paths through the wisdom of one or more philosophers and there was a real diverse system.
So, there is No Single (Human) Founder for Hinduism and it existed from time unexplored, as a religion.
For that purusha the celestials are the (first) worshippers. – Vedas