Hello!
I don't think that finding 40 suitable subreddits to share an Udemy course is realistic. I can find suitable subreddits and post on them, but I doubt there are 40 of them (there might be many subreddits with low subscriber count, and that would be a waste of funds in my experience).
So, I can first do a research on your niche and deliver a list of high quality subreddits for sharing. That would be the first milestone.
Second milestone will be posting your link on those subreddits (If your link isn't flagged for spam - very important!).
Third, optional milestone would be boosting one or two posts with 30 upvotes so that more people see it.
I think you won't find a better solution than mine, and I can deliver high quality results.
Let's discuss in details in messages.
Best regards,
Adam Radivojevic