one doesnt need it as far as we kno. only madara has gotten the rinnegan and he has poor stamina, no way in hell did u not see his perfect sussano and he fought with hashi to a battle of the death so he had a very good chakra reserve so ur idea of them only using a few rinnegan techs are absurd. look at sasuke now he can use a jutsu such as sussano and amaterasu with no visible strain dont compare the uchihas with MS which is known to take a lot of their stamina and eyesight away but ems makes it a lot more efficient. itachi had a disease and used the MS what do u think would happen to him and he still used a bunch of techs against sasuke in the fight. Uchihas should be able to use the rinnegan but obviously not to its full potential but should be more efficient with their dojutsus than outsiders cae and point kakashi.
I am not disagreeing that senju's power doesnt make them a hell of a lot stronger but its not necessary from what we kno about the rinnegan
It's clear as day that in order to have to power of the Sage you need both halves. Also arguing that Nagato wouldn't be able to use a sharingan or progress it is hilarious as Kakashi and Danzo are proof that you can in fact use and progress Sharingan without being an Uchiha. it only doesn't come as easy mostly because normal people don't have to quality and quantity of chakra the Uchiha's spend on their taxy doujutso.
On the other hand you stated yourself that Madara must have been a beast on his own if he used that Susano'O against Hashirama, not only in their final battle but every time they clashed Hashi was only superior by a little. Meaning Madara was a chakra beast at the same scale of Hashirama, to be able to keep up. And this same reason makes the argument that Madara needed Hashirama's cells for chakra crumbles. What was said was that
after he got Hashirama's DNA on him, reason why they clashed at the valley of the end according to Madara, he was able,
thanks to said action, to
eventually progress his
EMS towards Rinnegan.
Note that he actually only obtained the Rinnegan close to his death. And from the info we have that was about 20 years (Counting from the Valley of the end up until 30 years ago, that I reckon should be the age of Nagato) if it was a matter of getting chakra fuel then it would have been immediate. But it wasn't it was a process of fusing that must have taken a long time to prepare and experiences. So all points that Senju DNA is a must to get Rinnegan, and not only their amount of chakra.
Bare in mind that what the Younger Son inherited was the body and that's half of the sage's chakra but also the DNA that incorporated the Rinnegan was the Body's DNA.
Try to think of this as the computer where you are reading this right now. Senju's DNA would be the instructions to build the hardware. Uchiha's DNA would be the script of the software. Without the two you can't open Safari and read NB (Rinnegan).