Platform recommendation for an online competitive programming school I am supervising a team that is currently trying to build an online competitive programming course/school for a collection of high-schools in Turkey that all belong to a certain private educational institution.
As a former graduate of one of these high-schools, having competed in physics Olympiad and currently studying CS I have been offered this job along with other college students studying CS, some of whom have won medals in national and international Olympiad in informatics.
Our team has already made an outline for a curriculum and started teaching to a group of c. 80 students over Zoom.  Although we are confident in how and what to teach, we are unsure which platform to use as an LMS for uploading, sharing and organizing course material like presentations, assignments, etc. As a provisional solution we are currently using Discord but  are aware that in the long run Discord is not optimal as an LMS, as there is no way to share and organize course material in a structured manner. We are considering a better and more professional solutions but having very little experience in this area we are unsure which option to choose. Following have been considered:

Microsoft Teams
Moodle or a similar LMS
A custom build website

I have been told that moodle is difficult to set up and is an overkill. Developing a custom web-site seems not very practical too. Do you think microsoft Teams can be suitably used as course/learning management system, where course material and assignments can be managed in a structured way? Do you have any other suggestions?
My first instinct is that any LMS is overkill.  LMSs manage classrooms, it's true, but they also are designed to help manage schools, and most of the administrative end (and most of the features outside of the literal classroom aspects, and even many of the features within the classrooms) of any LMS would be completely useless to you.  Busing, medical designations, learning disability plans, school lunch designations, guidance counselor assignments, etc, etc, etc, etc, etc, are all beyond what you need for such a small program.
I'm assuming that most of what you're looking for is a way to distribute and receive assignments.  If this is, indeed, the case, I actually recommend using GitHub.
At the start, kids can simply use your repo as a way to download files through the web interface, and can submit early assignments through Discord, just as you've already done.  After they're comfortable with the GitHub interface for downloading, you can start having them fork projects.  Introduce git functionality slowly, and keep supporting them with Discord in the meantime.
You'll get two big big benefits out of this approach:

You don't have to get into anything nearly as heavyweight as an LMS.
The students will slowly become familiar and comfortable with basic git actions.