Aha, found the gremlin!
I discovered now that with unlocked phones, after upgrading, it overwrites the APN values (under advanced/tcp-ip) with the branded ones. So the T-Mobile APN settings came back when the device was upgraded. Once I disabled the APN or edited it back to my carrier, the wifi no longer crashes!
(note: if phone is simlocked, you cannot edit these APN settings, bastards!)
I discovered it while leaving my unbranded 9700 all night on wifi but no sim, but the second unlocked 9700 with wifi off with sim. Initially I thought maybe some app was flooding my router, but now it seems the T-mobile UK APN settings cause this issue.
So if someone encounters something similar than me, check the APN settings. And if you upgrade an unlocked phone, make sure to check/re-edit those APN settings !