Developers of MOST Lemmy apps tend to be rather responsive

I have a version of Thunder compiled for desktop, and it's surprisingly easy to change the number of columns for Tablet Mode. There's probably other places too, but the line I changed (from '2' to '4') is here

Screenshot:

Full-size image link: https://postimg.cc/hzsFj4rj

I realise you might not want to compile your own version, but it suggests that the developers could add 'number of columns for tablet mode' option relatively easily.