Soft returns in giustified text - How to do it?

As noted, soft returns (line returns, line breaks) are to be avoided in InDesign whenever possible, especially in anything like body text.

It's not that they should never-ever-ever be used — sometimes, especially in headings or text pulls, they are the only formatting option that works. But they should never be used to format running text, no matter how common the practice is for authors or Word users.

What you need is a Paragraph Style for each element of the layout you want. If you have the first half of a paragraph that is supposed to break a line on the way to the second half, that's two styles. Especially in poetry or other structured text, the correct approach is to build a careful hierarchy of styles that allow such breaks, and adjusted space above and below, to create the layout that a line-return does.

Does that point you in the right direction? Questions welcome.

Why don't you want to use paragraph returns? And why would you use soft returns? As you have seen, you can't use soft returns in justified text, and in a general matter, you should avoid to use them anyway.

If you use soft returns to send a part of text to the following line, don't do this. Instead, use the “no break” feature to keep together the words you want to stay on the same line, and better, use a character style to do this.