I would not do either. It is fine to germinate them in a bag but don't put them into the ground. Put them in a small container and when they are big enough, then put them in the ground. Your seed starting mix looks fine for starting seeds but you do not have anything with NPK except the vermi compost. Vermi compost has a low percentage of NPK per volume. This is fine for very young mangos but you will need a much higher NPK ratio as the tree grows.
The larger the container the more difficult it is to maintain the correct moisture content. Germinating seeds in a plastic bag is OK but I would not do it because it can get too hot very easily. I would plant the seed in a seed starter container which is about 6cmx6cmx6cm, a little bigger or a little smaller is ok. I would keep them in this container until the plant had 2 sets of true leaves and then I would put the plant into the ground or into your 20cm container and let it grow there for a few months before transplanting it to its permanent home.
Growing mango seeds sounds like a cool project. For germination, it's usually recommended to start them in a plastic bag with a damp paper towel until you see roots sprouting. Then, you can transfer them to well-draining soil. Your soil mixture sounds pretty good with the cocopit, perlite, pit moss, and vermi compost. Just ensure it's not too compact and allows for proper drainage. Give it a go and keep an eye on those mango seedlings, they'll be worth the effort!
