just need to be able to select certain parts of a text message to copy and paste

I can copy and paste ? I long press then top right menu icon has copy

Just because it doesn't exist in other messaging apps, doesn't mean it shouldn't exist in Textra. A possible way to make it work, would be to make selecting a two step process. First, you long press the message to bring up the usual select view. Then you long press again on a word and the cursors appear to select the words you're interested in.

I haven't seen any other messaging app that lets you select only parts of a received/sent SMS to copy; all of them only allow you to copy the entire SMS.

In Textra, if you long-click a message then press the menu button or three dots in the top right, you will see an option to Copy the message. You can then paste the message in the text editing box to select only part of the message if you need to. So copy/paste definitely works in Textra. It just takes one step longer than in the stock messaging app since you need to press the menu button after long clicking a message rather than just getting a popup window with options.

Definitely! Adding some word processor utility function would make things a whole lot easier, but only change for the better, not just got the sake of change.

Because of this lack of functionality, Textra is useless for people that uses SMS-based tokens to authenticate against company apps, bank apps, wifi-hotspots on airports etc.
I love Textra, but I'm going back to stock or another SMS app that have copy/paste of words functionality.

you can copy a whole text by long-pressing and selecting copy from the menu. if i only need part of the text, I either paste it into the compose box or select "forward" from the menu. then you can select just the bits you need. (and just delete the "new" text you just started.)