@aa-g7u6kkby I know that this is an older thread, but I am currently using Opera GX v. 109.0.5097.130. This is my first time using Opera for a browser and I think it is great! This suggestion still works in 2024.
I use StartMe as a new tab/home page, and usually after installing the extension in a browser it will automatically configure itself as the default new tab. In Opera GX it eventually says that "Opera doesn't allow its users to change the default New Tab page. We suggest you add start.me to the Speed Dial."
Changing the keyboard shortcut for StartMe to "CTRL + T" and leaving Speed Dial without a shortcut or a different one does work though and without hassle, thank you for this!