I grow a few begonias but my success is mixed. Rex types seem hardest for me to keep alive.

I have a rex still in its original 4” nursery pot. It dries out very quickly after watering, so I have a feeling there is very little soil left in the pot. It is MUCH bigger than when I bought it about 2 years ago, and very full. Some of the leaves fade but I don’t know why. Older ones, maybe? It gets afternoon/evening sun. Once the tree outside the window leafs out, much less. I used to try to keep them in indirect light, but then they always died due to
mildew.

I don’t want to kill this thing that now ranks as my longest living rex, by ignorance. I have a feeling I should pot it into something larger, but I know some plants actually prefer to be rootbound. I really don't know what these like.

If you think it needs repotting, what kind of pot to they prefer? Plastic, glazed ceramic, or terracotta?

Thanks!