Don't discount
Blue Iris so fast, especially if you are going to buy another DVR with BNC connectors that may or may not work with Roku.
You could use your existing DVR to feed the cameras into Blue Iris.
Do like many of us do and purchase a refurbished computer that is a business class computer that have come off lease. The one I bought I kid you not I could not tell that it was a refurbished unit - not a speck of dust or dents or scratches on it. It appeared to me like everything was replaced and I would assume just the motherboard with the intel processor is what was from the original unit. I went with the lowest end processor on the
WIKI list as it was the cheapest and it runs my system fine.
A member here just last month found a refurbished 4th generation for less than $150USD that came with Win10 PRO, 16GB RAM, and a 1TB drive. More than sufficient to run what you have. Probably can't get a DVR much cheaper than that. And then this future proofs you if you decide to upgrade cameras later.
A DVR is an underpowered computer and with the Blue Iris computer you turn off Windows updates and the uptime will be comparable to a DVR. If you are concerned about power consumption, according to my kill-o-watt meter, my computer for BI uses less than my DVR does. In fact, for the true test, I actually still have an old DVR going just for kicks and backup until it dies. We had a power outage recently and the BI computer lasted the entire outage on backup power and the NVR did not - two separate backup units but the exact same model purchased at same time. You run the computer without the monitor on and BI runs as a service and you don't run anything else on it and the power isn't really as much as you think.