Need a guest network for home devices

"Guest network" sharing functions are determined by your router's settings, so you will need to consult its documentation.

Are the Macs safe if I connect our other non apple devices to the same network

It's not clear to me what you mean by "safe" but if you want information on your Macs to be available to other devices, you need to explicitly enable Sharing services on a Mac.

Conversely, if you want your Macs to remain off-limits to other devices (whether on your own LAN or not) then leave those Sharing services disabled.

For an introduction to file sharing services read How to connect with File Sharing on your Mac.

Rather than using a guest network which seems somehow to keep cutting off regularly, I will just set up two separate networks - a 2.4Ghz and 5Ghz with separate names. The 2.4Ghz will be for older non apple devices and the 5Ghz will be for Apple only devices and also they will be the ones which use Apps and will go on sensitive websites like banking.

Not best, but as options go it's ok.

It's a much better idea to use a simultaneous dual band router that is also capable of creating a "guest network". I know nothing about Asus routers but it sounds like yours ought to be capable of doing that.

The simultaneous dual band aspect of it bridges the 2.4 and 5 GHz networks—not to be confused with "guest network" which is not bridged to the other.

The reason is that it's best to let your Mac decide which network and which of their available wireless channels offers the optimal throughput: 2.4 or 5 GHz, enabling it to nimbly change from one to the other as required in response to ever-changing wireless conditions. For that reason the 2.4 and 5 GHz networks need identical names. No matter which one it chooses though, it will be separate and distinct from "guest network" (which also probably offers simultaneous dual band capability).

"Guest Network" should obviously have a different name.

A mesh network is the way to go these days.

So now would be better to just keep both frequencies with the same name so the device chooses the best network for it and keep a separate guest network for other devices (non apple).

Yes.

I understand the control aspect of wanting to know and constrain which network your Macs will be using and whether that's 2.4 or 5 GHz. However, it really, really is a better idea to enable both, provide identical SSIDs, and let your Macs decide which is "best". I would not have believed that myself had it not been for an Apple wireless engineer explain the reasons to me. That was years ago and I've long since forgotten those reasons.

Another reason for doing this in the past is I thought the slowest device slows other traffic using the same wifi network.

I think you may be recalling the behavior of a network that had to serve 802.11b/g-only capable clients in addition to newer 802.11n-capable devices. That hasn't been a problem for a long time, and even back then it was a problem only when those legacy were actively communicating. You created a mesh network, using 802.11ac, and that's the best option there is today.

"Guest Network" on an Apple AirPort Base Station (for example) is completely isolated and separate from its usual network, and I surmise the Asus does the same thing.

As far as your Macs are concerned, the only information about them anyone or any thing using your LAN could determine would be their mere presence, and even that is not possible if they are connected to separate networks. For example, I have no way of determining their existence from where I sit, nor does anyone else—even if they were to successfully "hack" your PS4—unless those Macs were also connected to their Guest Network. It's just a separate network.

However, if you were to connect those theoretically "hacked" PS4 or other devices to the same network as your Macs... that's a different matter altogether.

Also, even if you were to enable Sharing services, a remote user would still to provide login credentials (user name and password) for access. At some point in the distant past it was possible to create a User Account with a blank or null password. I don't know if that's still possible, but it was never a good idea to begin with.