Would it be better to replace dropbear with openssh if I intend to sftp files to a usb hard drive connected to my router quite often?
It does not matter. The speed is the same either way.
Better solution would be if you use NFS or SMB to share that USB to your PC and have it like a folder in your system. This way is much more easier 
Dropbear already relies on OpenSSH for SFTP.
What you install for SFTP support is a binary built from OpenSSH source code.
So your question is moot
Is this the right guide and package?
openssh-sftp-server
https://wiki.openwrt.org/doc/howto/sftp.server
NFS/SMB would achieve the same speed as SFTP?
Yep, like I said, you just install the OpenSSH binary.
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for me SFTP is slow compared to FTP.
try to use arcfour encryption, is the easiest on cpu, ftp is not using encryption, maybe that's why you have lower speed
Why are you comparing apples to oranges?
It's like saying 'hey by plane I go faster than by car'.