Hi!
Once I was studying both types of characters, but now I'm studying traditional characters only.

According to the statistics, I can write many more words than I can read, which doesn't make much sense to me. Are the words in traditional characters separated from those in simplified characters for writing, but are treated as the same thing for reading? And can I somehow selectively delete the progress in simplified characters (as in nuking, but keep the progress in traditional)?

It's counting the writing parts extra when they are different between simplified and traditional, but it only counts the reading, tone, and definition once, because they have the same answer (and so are treated as the same "item" in the datastore, even though the item may choose simplified or traditional to prompt with). Not perfect and a little confusing, but we don't have a better solution.

There is no way to actually remove progress except for nuking, and there is no selective nuking. So I'm afraid that the extra simplified characters are going to stay in there, making your writing counts larger. I would look at the totals for the other parts as I were you, as they should be similar to what your traditional-only writing total would be.