Davinci Resolve, Fusion tab: how to properly resize pictures

Full question: how to properly resize pictures so that they look the same no matter the timeline resolution?

There is a resize node, but it breaks aspect ratio no matter what.

It looks like desired effect could be achieved using Auto resolution and Change pixel aspect options (in the resize node), but it doesn't update aspect ratio when I change resolution of the timeline. If I disable and enable again Change pixel aspect option, it fixes the aspect ratio, but it's a manual process, and I'm looking for a way for Resolve to resize the picture automatically according to timeline resolution, without breaking the aspect ratio (which is different from the timeline aspect ratio).

I'm trying to edit in 1080p and change it to 4k before exporting, but I have to make all the pictures to look the same no matter the resolution first.

I haven't ever tried to do this, but it might be helpful to start with the assumption that Fusion works best at a fixed aspect ratio. Therefore, I'd work in a Fusion composition that has enough vertical and horizontal resolution to cover all of the eventual output formats I want to produce. So if you want to be able to export HD footage in vertical (1080 x 1920) and horizontal (1920 x 1080) formats, I'd use a 1920 x 1920 Fusion composition. And then on the Edit page I'd set the clip containing the Fusion composition to have the Retime and Scaling -> "Scaling" option in the Inspector panel to "Fill" so that you end up with the composition filling the horizontal or vertical frame even after you switch the aspect ratio of the timeline.

There may well be a better or more flexible way to do this, though, so I'm all ears if anyone else wants to chime in.