I’d be very surprised if your isp router did not have at least a basic vlan and firewall but that is all I use, separate vlans and then firewall rules to allow my phone to communicate with the iot devices.
Routers can be virtualized in your server or if you want to buy a separate device anything that runs openwrt will work, doesn’t have to be a huge expensive one.
Upgraded the router that firewalls all of my stuff from the other adults in the house, and the replaced router (gl.inet Mango) is now free for various shenanigans.
Bought a pack of 8266 boards with various led and breadboard accessories to play with, and an air quality sensor that will hopefully end up sending data over the home automation vlan.
Spent enough on cloud storage to get the ‘3’ of my 1-2-3 backups done for all of my photos and videos, not automated yet but that is on the list, plus picked up a few very inexpensive usb storage drives for friends.
Also bought a heart rate sensor for cardio training, and keep thinking of ways to sniff data off of it for analysis, though that’s probably not the greatest idea…
Can be expensive, can also be cheap. I enjoy the five dollar arduinos and twenty dollar single board computers just as much as the high power machines.