可愛すぎる「ひかり」のイラストを見たとき心キュンキュンして褒める言葉を伝えなきゃいけなかった。

There’s nothing wrong with the original sentence, but a noun phrase is often preferred as a title. For example, some light novels are titled something like 俺の妹がかわいすぎる件, where the “件” turns the whole sentence into a noun phrase. If you want to do the same with the original sentence, you could say something like: 可愛すぎてキュンキュンした「ひかり」のイラストを見た時の褒め言葉.

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投稿：可愛すぎて苦しいんだよな！

As you probably know, ending particles are all about the tone or vibe you want to give, and each one has a slightly different feel. I’m not sure if you specifically chose “だよな”, but here are four possible ways to express it. I’ll do my best to explain:

1. 可愛すぎて苦しい！: A simple exclamation; this is probably what I’d use.
2. 可愛すぎて苦しいんだ！: Adding んだ emphasizes the statement and makes it feel a bit more dramatic.
3. 可愛すぎて苦しいんだよ！: The よ makes it more assertive, like you’re really expressing it strongly.
4. 可愛すぎて苦しいんだよな！: The な adds a sense of seeking agreement, like “you feel the same way, right?”

1–3 describe the feeling with different intensity: 3 > 2 > 1. The 4th one feels a bit different because it’s more about seeking agreement from the listener than just expressing the feeling. I feel like 1 fits best here since it’s not too dramatic, but that might just be my personal preference.

表情や姿も、すべては完璧。

”表情や姿も全部完璧” feels a bit more casual.

ひかりの優しくかわいい性格もうまく表されたんだ。

The previous sentence is in the present tense, so I’d keep this one in the present tense as well. : ひかりの優しくかわいい性格もうまく表現されてる(or 表されてる)

(これで)決まったんだよ。

I’d go with the present tense here as well.：(これで)決まり！

可愛すぎ。「ひかり」のイラストを見たとき心キュンキュンして褒める言葉を伝えなきゃいけなかった。

表情や姿も、すべてはが完璧。

ひかりの優しくかわいい性格もうまく表されたてるんだ。