I speak conversational Japanese, have been translating songs for fun for at least 10 years, and translate songs for commission for Vocaloid into Japanese. I only ever hear the "u" in desu pronounced noticeable in anime for certain types of character tropes. I haven't noticed any actual normal conversation (in person, on daytime TV) being spoken with a noticeable "u". (Not that someone doesn't over-pronounce it, I just never heard a real person say it.)
"Desu" means "is/am/are" but also adds politeness to i-adjectives. "Da" is the plain equivalent used for building grammatical sentences (such as connecting 2 sentences into a large one) or speaking to close family/friends/yourself.
You only hear "da" instead of "desu" in songs, because "desu" is too formal for songs. Sometimes you hear -masu verbs in songs, though. But almost always, you need to use informal speach for songs unless you are writing a character song for an anime character CD where their "thing" is that they ALWAYS talk extremely formal (ex: a butler character or something).
In Vocaloid, I just write"_0" after the phoneme pair to create the devoiced sound.
@parallax_fifths the o vs wo happens when the line is very emotional or if the preceding word makes it so that saying "wo" makes the pair easier to pronounce.
I can answer other questions if you have it.
Here are some general song-writing tips:
You could consider ending some of your lines with "de" (this is da in the ~te form that implies "and").
It is also good to not just end all of your verbs in the dictionary form (ex: ~ru), you can write them in ~te form to loosely connect it to the next sentence (ex: mitsumete instead of mitsumeru (gaze)), or in a poetic ~masu stem that doesn't attach ~masu (ex: Instead of writing motomemasu, say motome (search)).
You can also avoid saying "da" (since I said don't use "desu") by arranging the verbs with the nouns in a different order. Like... instead of saying "kimi ga suki da" (I like you), turn that noun into an adjective! So like "suki-na kimi" with no "is" at all. Or instead of "kono machi wa shizuka da" (this town is quiet), use "kono shizuka-na machi (de blah blah blah)".
The more lyrics by other people you memorize, the easier it is to write lyrics that "feel" Japanese. Try to not write the lyrics in English and then translate them, try to write then in Japanese FIRST and then write an English translation later, this produces more natural Japanese (since some sayings are weird in Japanese). I think that ending sentences with a verb or implied verb (where the sentence just ends like "hikari o" or "kimi e") is more common than ever hearing "da/desu", too.
(Sorry I wrote a lot, i just REALLY like Japanese and I REALLY like lyrics.)
Edit: You could also consider saying "blah blah da ne" or "blah blah da yo" if you have to end the sentence with "is/am/are", I've seen that type of copula + particle pair before when the singer is talking to the listener.
