I am not (even slightly) a ubiquity fan.
That said, for your 2.4 GHz dedicated SSID for your reef, jus about any AP will do. I suggest Aruba even though I despise HP products. Something small like an Aruba 203R or AP11D etc.
Ruckus makes good stuff to.
Cisco 140AC
NETGEAR AX4200
You must buy an access point, not a router/firewall with WiFi.
setup tHe SSID as single band/radio 2.4 amd turn off 5 GHz
Use a different SSID than the other networks on your home.
pick a clean channel
setup close to P4 if possible and hardwire to your existing network.
All of that said, the node hopping is more the issue than having a 2.4 and 5 radio broadcast the same SSID… a 2.4 only device can’t even see the 5 GHz radio. Where the 2.4 and 5 shared SSID issue comes into play is for devices that do both and constantly band/radio hop between nodes or radios. (Example a modern XBOX)