Here, 則 means "then". 却 = "however". 目标远大则成就伟大 = When your sights are grand, THEN your achievement is worthy of admiration.

we can't replace 则 with 却here. because 则，we use it when a good thing has a good influence or bad thing bad influence, as for 却, it's normally a optimistic condition but has a bad influence

Agree with Jeff: that's means :then

则 and 却 are not always interchangeable. 则 has many usages. Here, it is not used to show contrast. I could be wrong, but I'm fairly certain.

目标远大却成就渺小。目标渺小却成就伟大。却是前后句反转时候用。