Hmm, well pfSense does have the ability to send different boot files for different client types so you may be able to do that:
[image: 1666659180710-screenshot-from-2022-10-25-01-52-30.png]
Otherwise you would need to arrange some sort of override for the auto-generated conf file. I thought there might be something build in for that, like there is for mpd.conf, but I cant see anything.
Ancient but would probably still work: https://happy-coder.com/2014/06/27/pfsense-custom-dhcpd-configuration/ YMMV!
If you do find something that works you might drop a reply here as that looks like the same problem: https://forum.netgate.com/topic/174712/ltsp-on-vlans-pfsense
Steve