Not a bad idea.
but for me till i read that i considered all of it as tutorial. means its directet towards new players...
for me the last one was hard, but i allways had the feeling i can do it.. wich after trying diffrent stuff worked out.
1st try was just exchange broadsides, dont do it.
2nd try was run and demast with stern cannons, tried to abuse the fact that in a chase the cerb dont have bow chasers (took to long)
finall try was chainshot one, isolate the other sink him, repair and board the second with the sail dmg.
so for myself the difficult level was enjoyable, but that feeling i can do it may not be the feeling new players get from the challenge.
so, one idea is, if u see the later steps (challenge, finall exam) as not part of the tutorial, dont include it there, rather make it a seperate challenge for rank2/3 so that players that try those have alittle bit more experience.