My experience is very different !
Coconut coir is a real pain and struggle to remove and separate from the roots when you need a transplant.
Likewise small plastic containers are way too small for growing trees especially from large and medium seeds like nuts or mangoes.
Ideally, you want to pick a really big one (at least 5 to 8 liters) with a preference for depth over width, to grant the tree more space to grow and not damage the roots. They are quite sensitive in the first year, and each transplant will slow the growth for 1 to 3 weeks.
Growth is driven by two things : environment (sun/water/temperature) and soil. The one you find in most gardening shops is fine, as long as it's not too heavy and doesn't contain peat (might be difficult for our North American friends). I switched to fine, light soil with mycorrhizae and wood fiber, with a greater success.
Fertilizer is completely optional in the first year, but if you add some, avoid putting it in contact with the roots.
Finally, trees grow stronger if they are kept without support under the assault of the rain, wind and sun. They are meant to resist and adapt to their environment, so you really don't need to add a prop to keep them straight.
This year, I'm testing different style of pruning. My intuition tells me that no-to-very-light pruning yields better development than the fast and heavy pruning you see in gardening shops.
PS: I've grown 30 trees from seeds on my small balcony.