I can’t find anything re the Presto app and Airplay, so unless someone here had experience, you may need to email their support department and ask them what file format and resolution is used when the app uses airplay 2 to cast to a device such as WiiM. Note that Airplay 2 is likely to be the constraining factor, not the WiiM device per se.
In order to get the full file resolution, Bandcamp or Presto would need to be built into the WiiM Home app - I suggest you submit a ticket to WiiM asking them to consider such implementations.