As others have mentioned VLAN is an opt-in step and requires support from the hardware.

I also DONT want those devices to be able to connect to the internet: I have a D-Link webcam that has (had?) some issues around being exploitable remotely via the MyDLink or whatever it‘s called service. I don’t want to have (more) devices sitting in my network that open it up from the inside.

Also I need my guests should be able to access the internet without me having to whitelist their MAC address.

It would but most consumer devices don't support VLANs and if they did no consumer would know how to configure them.

If you do need low cost VLAN-capable gear I've had success with TP-Link switches and Mikrotik WiFi APs.

Yeah, what dborham wrote: most people wouldn't know a VLAN if it bit them on the nose, but if your your router automatically created the SSID "foo-devices" to go alongside your regular "foo" people might use it.

Like the "offer a guest network" button which, now I think of it, might already be enough.