In case anyone’s still interested, I just updated my own fork to display quite a bit larger cover image on landscape iPad plus added a feature where tapping the cover image hides the play controls and moves the cover image to the center. Another tap reverses this. This cover art feature works on all sizes and aspect ratios.
Still available here: GitHub - miemo/roon-web-controller: This is an extension for the Roon music player that provides a web based remote. THIS FORK CONTAINS VISUAL TWEAKS ETC, SEE TOP OF README FOR DETAILS!
Works great even on my ancient original iPad Air running iOS 12 