for https://wordpress.stackexchange.com/questions/396854/gutenberg-button-border-radius-default

theme.json

{
   "version":1,
   "settings":{
      "color":{
         "custom":false,
         "customGradient":false,
         "gradients":[
            
         ],
         "link":false,
         "palette":[
            
         ]
      },
      "spacing":{
         "customPadding":true,
         "units":[
            "px",
            "em",
            "rem",
            "vh",
            "vw"
         ]
      },
      "layout":{
         "contentSize":"810px",
         "wideSize":"1100px"
      },
      "blocks":{
         "core/button":{
            "border":{
               "radius":"22px"
            }
         }
      }
   }
}