Drives can and do fail sometimes. I'd sooner do 6/6tb than a single 120. That's a big expensive door stopper if the 120 fails at some point.
I'm sure BI is superior to an nvr and it seems like there's integration with your smart home stuff which is what you wanted. But your initial post mentioned quickly, lol. This can be the rabbit hole you go down when choosing a computer. I had my nvr and cameras from Andy installed and running in six days, including from ordering to killing myself installing the cameras. It doesn't integrate with Alexa. I have some ring stuff for that. But a quick setup of the nvr and there you go, off to the races doing what it's meant to do. I know me and if I either bought or assembled a computer with good specs I'd be selfish and want it for more than just going numb for security cameras, lol. If I'm gonna spec out a pc, it's gonna be mine, not the cameras. But I get it for sure. It's absolutely the way to go in a lot of cases. But, so can be an nvr.
Good luck with your setup. May you be bored as hell with what you record. The serious stuff can be bad, very bad.