Gave the built in DHCP server a go to see if I would run into your issue, and I did indeed with a couple IOT type stuff. After a ton of debugging, it turned out to be the fact the FastIron DHCP server is not set as authoritative, and some DHCP stack implementations (like Roku's and Sonos) do not like this, and will ignore it.
I went ahead and just spun up a really lightweight debian VM running isc-dhcp-server, it has it's own transit vlan to the switch, and the switch is set to relay all DHCP requests to it.
Gives me WAY more control than the built in dhcp server in FastIron or pfsense, and with glass-isc-dhcp you get really nice visualizations and lookup of all leases that you wouldn't get otherwise: Akkadius/glass-isc-dhcp
let me know if you want some help setting it up as well as my isc-dhcp config, it's as simple as just apt install isc-dhcp-server then copy in a config, and it'll serve DHCP requests to multiple routed vlans on your switch (just need an ip-helper statement under each VE on the switch)