Hi Billa,

re:
"just one big plane instead of multiple coplanar fragments"

That means that instead of this (here are 2 coplanar fragments):



For helping with fillets you instead generally want just one face there:



To do that quickly you can use boolean union when gluing the halves together instead of join. It is kind of a good practice to use Join when you don't need pieces to be intersected but here it would be good to have those planes fused together which boolean union will do.

- Michael