A router and 2 access points is outside the definition of simple if you want a guest network for the whole system rather than a guest 'port' per access point. This is exactly the strike in point for 'My First vLAN' and is an increasingly common home network scenario. I see no need to vLAN the basic LAN. Even if you add an IoT vLAN [again, increasingly common], I see no need.
Yes, this is clear enough as a basic howto, but it says "Although the master interface (wifi2) is on the router’s main bridge, it is important that this new slave not be on any bridge." So now we hit the problem that we need a new paradigm if we want to run the guest network seamlessly on both 2GHz and 5GHz. We could use the CAT5TV approach of adding a software bridge, but this is a dead end for more than 1 AP, where CAPsMAN and a vLAN is the way to go.
Re The 12 rules of Mikrotik club, it says Virtual LAN identifier or tag that is used to distinguish VLANs. Must be equal for all computers that belong to the same VLAN. BUT DO NOT USE explicitly VLAN id=1, imagine that ONLY 2..4094 are valid, unless you really know what you are doing. I always took this to mean that if you left the implicit vLANid=1 stuff alone and didn't try to limit, extend or otherwise alter the native vLAN, it shouldn't be a problem. I do understand that this heuristic might be limited to a Mikrotik monoculture and that works very well for me, even with some Netgear unmanaged network thrown in.