Here’s a sample app doing most of what you want, I think:
https://anvil.works/build#clone:WHOPZDIXUOXVGJOC=5VXZEEWMXZHPYWUFRUKIUDA4
You could also build the Data Grid in the Drag-and-Drop Editor first to more easily customize its look, but I did it in code to make it quicker for me. Either way, using numeric column keys lets you manipulate the resulting grid more like a matrix.