How to encrypt data and know it will be secure for at least a few decades?
Is this a good idea?


Not really.

It is rare to find attacks against vetted designs that allow a complete break with a tiny amount of cipher-text.

Assuming you plan to have a unique key (which isn't used anywhere else) for just this few kilobytes of plaintext, it's very unlikely any cryptographic attack could be mounted against any modern cipher that would recover the plain-text.

Honestly, your message is likely to be absolutely fine encrypted with AES.

It's very unlikely someone's missed a key-recovery attack on AES that can be conducted with a few kilobytes of cipher-text.


  Does the nesting increase the security, or does it somehow introduce a new vulnerability?


It is possible to do this badly and introduce a new vulnerability, yes.


  If this is a good idea, which algorithms would be best to use?


Just use AES in GCM mode.


  Would it be better to use asymmetrical algorithms instead of symmetrical ones?


No.


  How many iterations is enough?


One iteration of AES in GCM mode is enough.