以后 after (things happening after a long time) Sau này 然后 afterwards, later (things can happen soon after) Sau đó

以后 typically is used to mean "later" 然后 typically is used to mean "and then" .. Used in a sequence of logical actions. 以后。。。 吃饭以后我们走吧。Let us go after dinner. 然后。。。 我们先吃饭然后我们走吧。Let us first eat then we will go. 先 is used to set up the sequence of events. 以后 cannot be used in this situation. Based on what I learnt from https://resources.allsetlearning.com/chinese/grammar/Comparing_%22yihou%22_%22ranhou%22_%22houlai%22#.E7.84.B6.E5.90.8E_as_.22and_then.22

normally，以后means something has not happened yet or it might happen after a while. However, 然后always means it was likely to happen or it is going to happen in a very short time. Hope to help you. Here for your understanding, 以后we could be friends, 然后we are friends.