So...after scouring the web....
I found this:
https://www.reddit.com/r/PFSENSE/comments/9l5fqo/422_upgrade_boots_up_yet_monitor_stuck_at_booting/
I proceeded to test, and yes, pfSense 2.5.2 does boot in legacy mode on the Zotax CI-323 to install, you are just unable to see it booting as the console output stops after you see the message can't find file '/boot/entropy'.
The solution was shown in the above link to be:
hit [esc] during initial pfsense boot.
At the prompt, type "set kern.vty=sc"
hit return
At the prompt, type "boot"
I was able to see the boot and prompts to install once this was completed.
There is likely an issue with a kernel driver, but this solves the problem for now.
Once I installed with the updated realtek driver, I my /boot/loader.conf.local config is as follows:
kern.vty=sc
if_re_load="YES"
if_re_name="/boot/modules/if_re.ko"
The system is now booting in legacy mode and I have the updated CI-323 driver 1.96_04 installed as a module.
The system is stable.
For what it's worth, I have used a pfsense Zotac CI-321 for more than 2 years with a recompiled kernel and loaded the updated realtek drivers in the kernel. This box achieves 1Gbps throughput routinely and hasn't been rebooted for months. I run it on AT&T Fiber for basic firewall with NAT.
I will be recompiling the FreeBSD kernel to include the updated realtek drivers for the CI-323 as well with hopes of duplicating the performance I get on the CI-321.
It's a shame we have to do this.
I do agree with you. The Intel NICs are mostly painfree with respect to pfsense functionality.
Realtek needs some love to get working properly.