First of all, as I always say: Material Design is just a guideline.
You may or may NOT find the answers to specific proboems you may have. And sometimes, when it comes to UX the answer is quite dubious to say the least. However, good UX existed before Material, and will exist once Material is gone.
Now, it's true Material is quite generic on the stepper messages, which I think is a correct approach for modt cases. There will always be special needs, but of course they can't cover every special case, so you need to use your best judgment. However, Material offers something that may help in your case, see Steppers Page under the Stepper Feedback title.
As you will see in the video, their solution is to add transient messages after succesful completion of a step.
As for changing the messages of steps.... Yes, it can be done depending on how you define those messages and of course, barring testing. If the change of titles mean a totally new scenario, you'll probably confuse the user, after all a stepper is just an element with a binary logic step or action non-completed vs step or action completed. Material uses iconography and color to achieve this, and it's a good and proven UX technique.
By default, a stepper is just a.... Step. So the name for each step is
Step 1, Step 2, Step 3 ......... step N
Which of course can be shortened by using just numbers, like 1,2,3 and so on
So users know they have to complete a step. Thus, "step completed" or just a check mark is usually enough. You can describe what the step is about, for example "Step 1: enter your info"
However, in your case, you want to move away a bit from the rules, so the messaging should be something which binary conditions are related. For example :
"Start by growing a flower"
"Flower has grown!"
Which is clear enough and related to the task performed. Something like:
"Now Let's kill dragons"
Might be related to the whole scenario, but it's confusing, because the user has no idea if the flowers have grown or not! So you need to always keep focus on the binary states of the task at hand