Maybe I’m just not seeing it, but is Reply as a Linked Topic not an option on the mobile view of the forum?
It is shown as an option for me (Firefox on a spiPhone).
It’s the arrow symbol on the top left of the reply form, above the text field toolbar.
… which is exactly the same as on the desktop/laptop view.
In the first image, I have underlined in red the button to click on.
In the second image, I am showing what comes up when you click on it. The behavior is different on a mobile phone in that it doesn’t waste screen real estate within the menu by showing the currently selected option - and the positioning might be ever so slightly different too.
Thank you both! I’m just either blind or forgetful.