Getting DDOSed or hacked is very very rare for anyone self hosting. DDOS doesn’t really happen to random people hosting a few small services, and hacking is also rare because it requires that you expose something with a significant enough vulnerability that someone has a way into the application and potentially the server behind it.
But it’s good to take some basic steps like an isolated VLAN as you’ve mentioned already, but also don’t expose services unless you need to. Immich for example if it’s just you using it will work just fine without being exposed to the internet.