Hardware wise, are the drives SATA? I couldn’t get an answer at a glance. If you are running them mirrored, you can back off on the cost.
The three drives may ramp up the fan on that. Check to see if there are additional fan headers and add some intake fans.
/mnt/ is where I would put this. Only exception would be if I had a service I was providing, like a media player server, and then I might mount it at /srv/
Those 2008 disks weren’t great, but airflow advice at that time wasn’t great either.
I can understand how you got where you are, but my concern is that your raidz pools will get more slow as the shared bus fills. You might be able to mitigate this by distributing your pools across different root hubs.
I get the point that external devices will have less heat entrapment, but laying your components in your case to optimize air flow will keep your drives cooler and extend their life. Also, I personally have a lot of risk in my house due to children and pets and can’t imagine leaving my devices out like that.
What size drives and what quantity are you considering?
I know it’s not your question, but you might get much better results from a SAS card and adding SATA SSD drives. You can slowly upgrade an ATX system to meet increased power and space needs. I have done the same using truenas and it’s working great.
Continuing to add USB devices does increase the serial traffic per bus since it is shared. Increased traffic increases errors, lowering your bandwidth further.
I was expecting them to be more expensive, but $85/drive isn’t bad.