how to encrypt a hard drive: what are the pitfalls? what type of encryption?

I have virtually zero experience encrypting hard drives. I only encrypted an Artix Linux install once, but I didn't really pay much attention to what I was doing. The only thing I learned is that there is a significant wait for the hard drive to mount every time I boot, since it's extrapolating the encrypted content. At least....I think. Maybe there's another explaination. And that's why I'm making this thread.

I want some basic suggestions which type of encryption to use for an external hard drive? And what pitfalls could ensue? Like, forgetting your password, but that's one I can think of myself, so I'm hoping someone more experienced would know other pitfalls.

Also, what about mdadm/raid compatibility? I choose mirror in case one drive fails, I can still recover. In a single drive failure scenario, what complications can arise?

Also, is there some Slackware specific encryption methods that do not apply to other distros? Slackware is my primary distro, but I also like Artix.

Also, what tools are recommended to test hard drives of their reliability before committing them to encryption?

I want to implement this on a SARPI 15.0 install. I want to do full disk encryption, unless that's not a good idea.