I think the most flexible solution is to use an external drive - either a USB drive or a NAS. You don’t need to open up your Nucleus; it’s marketed as an appliance, so it should be treated as such Also, HDDs are perfectly fine for music in terms of speed. That would save you good money.
Regarding the USB switch, yes, the Nucleus is a computer and the USB port is just that, so I’d expect any switch to work.
One caveat is if you put the HDD/SSD inside the Nucleus you will have to format it then be able to use it…this means you have to copy all your music from somewhere to the newly formatted internal drive.
Personally I would NOT recommend running an HDD or SSD of a USB hub. For syncing a phone or plugging in a mouse or keyboard no issue, but 2.5" drives drives tend to demand more power than a simple hub will be able to cope with. A USB DOCK however with USC-Power might be fine on a computer, but that possibly wont work on a Nucleus
not always…some come with a 2 USB-A split cable one for data and one for additional power…A USB HUB that is non powered wont help you here as the limit is the power from one USB port. SSD drives typically use more power than HDD too