I’m not sure what sina is involved and, even more so, why you consider it chinam.
I think my post above to benignuman should answer your question, though it was Rav Elchonon who held that Zionism is A”Z: I don’t need to defend him and he obviously doesn’t need me to do so either.
Violating the oaths is the crux of the matter.
According to a Zionist web site, Rav Ovadia is “ambivalent” on Zionism.
As I wrote, many religious Zionists struggle futilely to explain away the oaths.
If you read Rav Ovadia’s actual positions as he’s quoted there, though, he is not a Zionist. He does not write that he advocates military conquest, considering it worthwhile to sacrifice lives CH”V for the State and he does also not write that he disregards the oaths.
Here is what he wrote:
“What is Zionist? By our understanding, a Zionist is a person who loves Zion and practices the commandment of settling the land. Whenever I am overseas I encourage Aliyah. In what way are they more Zionist than us?”
Notice that last line: he defines Zionism as simply fulfilling the mitzva of settling the land, which means he seems to hold that the mitzva is, to some extent, in effect nowadays. But he does not say you can violate the oaths by doing so. Everyone davens for Tzion (i.e. not the government, in case that wasn’t clear) in shemone esrei and also there were other yishuvim before Zionism and I don’t know of an opinion that those people violated the oaths; they simply went to live in Eretz Yisrael, no strings attached. I don’t see Rav Ovadia saying differently.
The Zionists have no answers to the oaths and the gross violations of those that Zionism was and is.