This article is based on valmiki ramayan. In valmiki ramayan it is mentioned that there was no swayamvar organized for Sita. The swayamvar story comes from ramcharitmanas
According to valmiki Ramayan, Ram, Lakshman along with vishwamitra go to see the dhanush at mithila palace, Janak told them about his promise, Ram picked up the dhanush and breaks. Ram and his brothers got married to Sita and her sisters when dasharath came with his other 2 sons with the consent of the elders.
The pushpavatika story also comes from ramcharitmanas
After the death of the mothers, the 4 sisters, at the time of lakshman samadhi, Lakshman went to sarayu and took samadhi, indra came and took him to heaven, everyone welcomed the 4 part of vishnu to heaven
It says that 4 brothers are 4 parts of vishnu. It doesn’t specifies that they are- sheshnag, shankh, chakra. the other 3 brothers went to sarayu took samadhi and and merged in vishnu.
In valmiki ramayan, there is no way between lav kush and Ayodhya army, lav kush sang ramayan during ashwamedh yagy
It is a long story so I left out some, these are some of the difference between valmiki ramayan and ramcharitmanas
In valmiki ramayan it is mentioned that in ram rajya no woman become widow so sita and her sisters died suhagan