Learn Dutch: Wat betreft het genre is de catalogus breed. - As regards the genre, the catalogue is broad.

Yes! You will often see both wat het genre betreft (split) and wat betreft het genre (together). They mean exactly the same thing. The split version is slightly more common for short nouns and mandatory for pronouns, while the contiguous version is handy for longer or more complex noun phrases.

Why does the verb is come right after genre without a comma?

The entire phrase wat betreft het genre acts as the topic occupying the first position in the sentence. Because Dutch is a Verb-Second (V2) language, the finite verb (is) must immediately follow it in the second position, pushing the subject (de catalogus) to the third position. You will sometimes see a comma written after the topic phrase, but it doesn't change this V2 inversion rule.

Why doesn't breed get an -e ending here, since catalogus is a de-word?

Adjectives usually get an -e ending when placed directly before a noun (like de brede catalogus). However, here breed comes after the verb is to describe the subject. When an adjective is used this way (predicatively), it never gets an -e ending in Dutch.

If I wanted to say 'As regards that, the catalogue is broad', how would the phrasing change?