Selecting a range of hex tiles within radius of main tile

Using a grid of hexes I generate, i can hover over any tile and it will highlight just fine. Now what I'm trying to figure out is how to allow for a radial selection of tiles around that hovered tile (brush size basically). Eventually allowing for a somewhat radial soft-selection gradient to allow falloff.

Each hex already knows about their neighbors, so currently I am hard-coding logic to handle a radius of 1, 2 or 3. I could duplicate that logic over and over to handle bigger radii but this is definitely not going to work for much longer, so I am trying to figure out a more efficient and cleaner way using loops/recursion/whatever.

Can anyone offer any pseudo-code or possibly a website that explains how to do this? Below is a (probably bad) attempt at what i think it would look like.

 OnMouseOver(){
     // This would select a radius of x hex tiles including center tile
     gameObject.GetComponent<HexTile>().SelectTile(3); 
 }
 
 SelectTile(selectionRadius){
     if(!thisTile.alreadySelected){
         thisTile.Select();
     }
     for(int i = 0; i <= selectionRadius; ++i){
         if(i < selectionRadius){
             foreach(HexTile tile in thisTile.neighbors){
                 tile.SelectTile(selectionRadius - i);
             }
         }
     }
 }

Edit: Im using this fantastic resource to help with hexes - http://www.redblobgames.com/grids/hexagons/ - so might there be a section on this page that may explain what i need and im just missing?