Elon currently owns assets of more than 250 billion USD and donating 200 million USD to Mr. Trump's election campaign is not too difficult for him. Additionally, I think Elon has bigger plans in supporting Trump, not just for the benefit of his companies. With his acquisition of Twitter and his support for the Republican Party, it seems that he is preparing plans to enter politics instead of just serving his business. I wouldn't be surprised if he gets involved in politics in the coming years.
No politician is trustworthy and we should not be quick to believe what they say, but their public support for bitcoin is notable.
It makes me wonder whenever Elon Musk will be donating BTC to Trump or just plain-old Fiat (USD). Especially when Trump has given his full support towards Bitcoin and all things crypto. I doubt Elon Musk would donate DOGE.
Regarding the OP's question, I think Mr. Musk is only into crypto to gather all of the attention from mainstream investors. This will help him take his companies to another level. Not because he's seriously interested in crypto/Blockchain tech. You can see why up to this date he's been speculating about a possible integration of DOGE into X, without making it a reality. It's all words, but no action. Once market prices go to the moon, he will reap the rewards while the rest will go "rekt" in an instant. That's the way it works. Only a fool will follow Elon Musk to make a crypto investment decision. Who knows what will be of Dogecoin once Elon Musk decides to abandon it?
I think Elon decided to support and donate to Trump not only because Trump supports cryptocurrency. As everyone said above, Elon is thinking about the benefits for his business or as I predicted, he also intends to participate in politics in the future. So I believe he will donate in USD and this money will also be used to advertise the election campaign, so USD will be most convenient.
Regarding Doge integration into X, I believe we will see it soon during this bull season. It's just a matter of time because if this happens it will create hype for Doge and it will be more perfect if it happens during bull season than now.
After all, he is a businessman and profits are still his top priority so don't be surprised when he focuses on profits.