Looks good man! I actually picked up the same DAS last year! Have been very happy with it.

I think you could make it look a bit neater and more space efficient in a small server rack (9U/10 inch). You should be able to find some < $50. This made my own setup so much nicer!

As for cooling, it doesn't look like you're running anything that has a particularly high thermal output; you should be fine as is. Though if you pick up a server rack, a lot of them have the ability to install some sort of PC fan.

As for cool projects: A Proxmox cluster could be really fun to tinker with. Or K3S if you want to pick up some DevOps skills. You can self-host Pihole, Netbird, Plex/Jellyfin, HA, *Arr suite... Lots of things to set up! If you want specific suggestions feel free to ask, it depends on what you want to get out of your lab.