I’ve just discovered GitHub - max-kammerer/orion-viewer: Orion Viewer is pdf, djvu, xps, cbz and tiff file viewer for Android devices based on mupdf and DjVuLibre libraries will open TIFFs. It’s not great for a folder full of individual one page TIFFs (as opposed to single file multipage TIFF), as there’s no quick way to move from one image to the next, but it will do me in a pinch.
It’s on the Google Play Store at https://play.google.com/store/apps/details?id=universe.constellation.orion.viewer but not in F-Droid as far as I can see.