The full disk encryption software that Debian uses is dm-crypt with LUKS by default it uses AES and provides you with an option to change your key size but by default its 256 bit.
It is very secure and is the trusted encryption software that is offered upon installation of several Linux distros, No you don't have the ability to change the LUKS full disk encryption password and yes this is a safe method for encrypting your entire disk, while there are several attacks that can circumvent your full disk encryption, they for the most part require physical access to your device and some mistake being made on your part and not something that just anyone would know how to perform.
If you choose to encrypt your entire disk then the only thing that won't be encrypted is the Boot Loader (which needs to remain unencrypted).
The fact that Truecrypt is no longer supported and has been deemed vulnerable by the developers themselves would most certainly deter me from considering using it or relying on it to encrypt anything anymore. Instead of just wondering if one encryption software is superior to another, start looking at the specifics of the software, what algorithms does it use, hashes, is it open source, is there an active community of developers who tireless work to improve and maintain the software? etc.
All of those factors go into whether or not the encryption software is truly reliable or not. The appropriate software for you is the one that best fits your threat model and increases your OpSec