Usb or sata, both are slow because the Mips architecture.
Use cifs or transfer over ssh.
Also, a router isn't a NAS... is to get small files faster.
>bundle package "routeros"
I agree that bundle is bad, since is a "bloatware" to most non-enterprise users.
How to solve? Netinstall it! Install the ROS through of all-packages.zip and select what you need.
It's a good to have, but yet isn't supported.
>There must be a lite version. Or maybe an ISP version and home version. Or something.
Neither Cisc0, Huaw31, *-L1nk, *wrt, Asu5 and Netg3ar have a version like that you said that I remember.
In my concept, you need to know the tricks to handle with each network brand and enjoy better that device.
Or stay mad with the brand and try to use a new brand to you.
If you don't liked that, change it!