Is there a way for other nations to peacefully be integrated into my Empire?
Is there a way for nations that have good relations with me and are military much weaker than me to be added into my empire, through diplomacy, so that I'll not suffer any negative consequences for conquering them and breaking treaties.
Client state leaves them independant, only paying tribute.
If there is such a way, what are the requirements for it?
Solution 1:
No, the only way too completely assimilate a nation, is trough conquest.
Like you said, the other option is to offer them client state status, but that's not what you're looking for.