I haven't used textra or chomp in a while so I installed it and looked for download options.
All I can see is the option to download to the gallery automatically. I turned that off. I manually downloaded an image from a mms I received. It created a new new folder in internal storage/ pictures/ textra.
I don't have a SD card installed so I don't know if textra would recognize it and offer it as a download to option. I didn't see any download to options other than the default folder it created in internal storage.
If you can't change the download location there might be other options available such as syncing the textra album to Microsoft one drive, Google photos etc.