Is there a way to lengthen how long the Displayed Text Message is displayed?

No, that's what transmission is for.

If you really want to use Display Text Message, you can continue to display it and add enough lines to erase the old one.

You could add a bunch of spaces or invisible text, that should do the trick.

Using return for extra lines is the best way, as it uses less string memory.

But while it is being displayed you cannot chat at all and that's very annoying...

You can't chat with invisible text either, if you want them to be able to chat, just space out the time that it redisplays it.

When it ask you for how many seconds if you just make that high enough it will do the effect you want. And people can still text and do everything. Just dont do the invis lines becuase it is so anoying.