Hello!

I am 15 and I play Chopin 1.

It's not that difficult and more than that some people say that is easier than some Mozart Concertos. Mozart is hard to articulate. It's easy to play a Rachmaninoff piece where you can make legato with the pedal and bring out beautiful colours but it's hard to play Mozart right.
I don't like Grieg. It's too weird. But you decide if you want to play it.
Schumann Concerto is tooooo hard to understand. Schumann, in my opinion, is one of the hardest composers and his works are probably easier (technically) than Rachmaninoff's or Liszt's but have hidden difficulties.
I don't know the Mendelssohn or Saent Saens but I know the Tchaikowsky and it's haaaaaaaaard.
My teacher heard me playing the Tchaikowsky (it was a knew concerto for me and I wanted to play it ) and said that I'm a fool. She said that even her teacher with 30 years of experience in piano played it with fear on the concert stage. It's a monster and if you don't have decades of experience you can't play it.
There are other concertos you can play like the Scriabin. It's not very demanding and it's a gem.
Rach 1 or 4 are also managable but Rach 4 is weird (for me) and underplayed.
Liszt 1 is not that hard but depends on your technique. I played it last year and was really easy.