I do not think your custom "Thank-you" page is built into Moodle. But this is not to say that it cannot be done. Adam provided one idea for you.
As another idea, you could make the very first item in Topic 0 be "Welcome to this course." This could be a Moodle page or a link to an external
URL. You might even be able to set this up so that the student must click on it in order to do anything else in the course (conditional
activities.) But it appears that you prefer to have this Welcome web page automatically open itself when the student first enters a course, probably being closed by the student when they click on an "okay" button or something like that.
When desires are not totally logical, these desires tend to not be included in standard products, meaning Moodle. But Moodle is open-source, and you might be able to figure out a way, or hire someone to do the coding for you.
You should browse Moodle's Tracker site to see if someone else has already requested this feature that you desire.