Hello,
That’s a lot to consider for just one thread - If you want to focus on the “when using Foliage Mode, only the static mesh appeared, with no textures or materials” you should respond in the thread you made for that issue.
There’s many ways to approach this, here’s an idea for one approach.
Start with just 1 tree. Put down a forest of them. Now focus on how they illuminate when the light hits them. What does their shadows look like? (Can you see individual leaves in shadows) - Are you ok with things looking terrible on mobile/switch/performance mode?
Through this, you’ll find what type of tree assets work best for you.
Also It could be good to just focus on 1 ‘view’ - hold CTRL and press a number key to set a view bookmark. You could then have a very precise focus area to iterate on when designing the forest.
If you want to post pics of your WIP forest we could talk about specific elements.