I get that My new Mac book pro has a 1TB SSD in it which is plenty for what I need to store on it the rest of my stuff ( photo archive mainly ) is on the AirPort Extreme which has a further 2 TB which I have partitioned into 2 one half of which I do the time machine back ups to.
How much extra do you need as 1 TB hard drives are cheap enough even for top of range stuff and at the end of it they still have to be plugged into the machine and Im not sure how much faster the Thunderbolt stuff is compared to USB 2, 3 or indeed 4 in the grand scheme of things.
Some sort of NAS drive with raid might do it but that doesn't get your requirements of thunderbolt
One of these would do it for you I suspect but at the price I won't be bothering
https://www.apple.com/shop/product/...2a68797f65e5f0ae809d990b531aeb50d38b4dfe52d11