Not all. The Bevern and Oels lines still exist at the time of George III's last daughter IIRC.
And his sister, Augusta, will be pushing even harder for her eldest son to marry the TTL Princess Royal (OTL she also did but Queen Charlotte but a spoke in that wheel rather quickly, since she loathed the Brunswicks and even warned her brother off marrying Karoline as a second-wife). Here, George might agree to a match, if only so that his grandchildren will inherit Hannover as well as England. Parliament was also not in favour of a Brunswick-Royal match, since they viewed Augusta's own match as having been worthless (although they might appease her with a second daughter).
A hilarious candidate for Prince Consort would be...Max Joseph of Pfalz-Zweibrücken (or as OTL, Max I, King of Bavaria). Born a Protestant (although his dad was already Catholic, to "get ahead" in the Habsburg army rather than from sincere conviction), he and his brother only converted in the 1770s when the Bavarian succession was in jeopardy. MJ's dad and Queen Charlotte's brother in Austrian service were comrades in arms. And MJ used this to swing an invite to the British court OTL (although George III denied the application of a passport).
But, if by the time the future queen's marriage is being debated, MJ's uncle (Karl Theodor) is still in good health, and his older brother and nephew are all blocks to the Electorship. British Parliament is like "sure, lets go with this one, he can at least come to England and live here" (OTL Max spent most of the 1780s either fighting with the Deux-Ponts in the US, thus becoming the first royal to set foot on modern US territory or living it up in Paris). Fast forward to 1800 and the future prince consort of England is now suddenly the heir to Bavaria and the Palatinate.
Oh the irony. Britain got rid of one German albatross and by tryig to be clever, wound up with another, Catholic one.