This is a Celeron dual core. Supports 64bit instructions. Yes I think DD-WRT 64bit will load into it. The problem you can run into is if the graphics card/chip is not supported it can cause a crash during boot. But there is only 1 slot for WIFI card to fit into. Which you can only choose 2.4 or 5Ghz on that mini pc.
For a regular computer like you have now. If you want the 2.4GHz and 5GHz you need to purchase a second 5GHz capable card and install both of them into the computer. So that is another mini pci-e to pci adapter you have to buy also. You can set the one card to 2.4GHz band and the other you set to 5GHz band. Yes those "laptop cards" only do 2.4GHz or 5Ghz not both at the same time. Most of your dual band routers you buy at the store actually have two radios in them. I never seen a wifi card do both at the same time. In my experience 99% wifi devices support the 2.4Ghz band I only have couple devices supporting the 5GHz band.
No, I'm not yet looking if I can put domoticz on it..
and I want Pi-hole on it
pi-hole for ad blocking? The browser addons like Ad block plus or uorigin not good enough?
As for home automation. I think the best way is to have a controller hub DMZ to it so you can access it from your public IP address or Use port forwarding. Instead of loading a piece of software in DD-wrt.
I do not have home automation yet but that might be in the future for me.