You cannot impeach a citizen, however that's not what's happening. Trump was impeached while president, that was why the big rush. Once jan 6th got here, they couldn't impeach because he wouldn't be president. It's possible that they also thought maybe he did have another trick up his sleeve and would stay somehow, doubtful but possible.
The trial phase is an interesting question since the majority penalty of guilty is removal from office. However I think the real desire of the Ds is what happens AFTER if he's found guilty. Once found guilty they will then move to bar him from ever holding public office, or receive fund raising, again. They are that afraid of him running again.
Honestly, I think he'd be better served, and the country would as well, to be more of a king maker than a king. In 4 years, he would be to old IMO, that would put him into his 80s at the end of his term (note I feel the same way about Biden). Trump can do more for America by continuing the message he's started, and push for a new republican, or another party if he can't redesign what republican means, and bring in a newer, younger face.
