Fixed or worked around.. They are completely different ;)
Source natting would not be a fix to me.. That would be a work around. To me the proper fix for your issues would be correctly setting the firewall rules on your devices to accept the traffic you want to accept. Or make the choice that devices on network X behind pfsense do not need a software firewall because they trust all the devices on their same network, and devices that are hostile or not trusted are firewall at pfsense.
To a nas. it should have a gateway set if that was your issue. Or if firewall - same thing goes. Tricking something into thinking a connection is from the same local lan as it to get around firewall rules and or lack of gateway is a work around if you ask me.
Either way glad I could be of help, but if you went the source nat method. I would would evaluate if that is the best long term fix vs stop gap workaround until proper setup can be used, etc.