VLAN configuration GS1920-24v2 and Nebula NCC

Hi @Saevik,
 "Is this bad for safety", it depends on whether malicious event exist and your network scale. More device will decrease your network performance.
 "Flood of Internet to both VLANs" means if you configure like you posted, VLAN 2 = Internet, VLAN 3 = PC 1, VLAN 4 = PC 2. If PC1 visit a website, PC2 will receive data from the website. Normally PC2 will drop the data and waste resource in some degree. If someone with bad intention may use it. 
Zyxel Nebula NSG is of course much secure for your network. And it is easy to use.

Hope it helps.