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This is a good guide: https://wiki.tnonline.net/w/Btrfs/Replacing_a_disk
Usually you want to replace drives before they fail (SMART monitoring will give you ample warning in most cases). The it is better to have an additional free SATA port to turn the failing raid temporarily into a three-way raid and use the btrfs built-in function to replace the disk in situ.
Could I purchase two different brand drives and use them with btrfs?
I don't quite remember the source for this, but I believe I read some time ago that it's actually a good thing to have separate drives. The reasoning is, if you buy two identical drives (at the same time), the likelyhood of both drives failing around the same time is severely higher.
This is then amplified by the fact that rebuilding a RAID puts a lot of strain on the non-dead drive, so if ie. drive 1 dies and drive 2 is about to die, the strain you put on drive 2 in order to rebuild your RAID onto drive 3 might kill drive 2 before you even finish rebuilding your RAID.
Again, this is just from my memory, it might be worth doing some more research on.
It is better to have more drives and less apares. However, btrfs is only stable in raid 1. With data that big I would go ZFS raidz2 as you can lose up to 2 drives.
From a btrfs perspective it is pretty easy as you just can run btrfs replace with the path of the new drive. Btrfs also has the benefit of being native
BTRFS is stable for all RAID levels except for RAID 5 and 6 (because of the write hole). I'm using it with RAID 10.
Is btrfs RAID stable yet? This article is three years old, so maybe things have improved, but it contains some pretty strong warnings about the dangers of btrfs RAID:
https://arstechnica.com/gadgets/2021/09/examining-btrfs-linuxs-perpetually-half-finished-filesystem/
To summarize, the article argues that btrfs is great for single-disk usage but its RAID implementations are idiosyncratic and unreliable.
(I use btrfs daily on several single-disk computers and it has been great, but I have never tried its RAID.)
Mirror (raid1) btrfs raids are fine and are more convenient anyways.
I believe it's only RAID 5 and 6 that are unstable, https://btrfs.readthedocs.io/en/latest/btrfs-man5.html#raid56-status-and-recommended-practices
Been using it for over a year on two 8tb SSDs in strip and 14tb as mirror. This is on Debian and its flawless and wonderful. I run btrkbk hourly for snapshots, backups to remote locations and house keeping with 6 months of hourly snaps. Life is great.