P: Improvement to Transform Upright Tool on panoramas

you should https://ptgui.com give a shot. here you can play with many different projections and force vertical lines in the image to stay vertical. usually, this works better than the PS tools. especially this bending you describe which occurs when taking a panorama capture with a positive tilt angle and which can be very difficult to handle in PS, can be corrected with ptgui very precisely.

This is because of the chosen projection in the panorama merge dialog. You will have to choose 'Perspective' to get straight lines, the other two projections create curved lines by definition, because they are projections inside a sphere or a cylinder. The Upright tools are not designed to correct that. Perspective projection is only possible for relatively small angles, however. If you cannot use it because the angle is too wide, then look at Photoshop's 'Adaptive Wide Angle' filter to correct a curved panorama. Or choose a dedicated panorama app like PTGui.

Have you tested out if using a different projection helps, perhaps the Perspective.

https://helpx.adobe.com/lightroom-classic/help/panorama.html