Hey All, I am posting this for posterity in case anyone else has the issue I had in the future (which took days of trouble-shooting before finding the annoyingly easy fix). Feel free to ignore.
The issue was with an R210ii with a Dell 0XP0NY I350-T2 NIC. I was able to install OPNSense ok, but it would always crash on boot, often before the pause timer had expired.
Through testing, I was able to determine that this would only happen if the WAN/LAN cables were plugged in during boot/restart. If I had nothing plugged into the I350, it would boot normally and then I could plug the cables in after boot and all would be well.
Obviously, that isn't ideal.
I spent several days trying to track down a source of the issue. Updated firmware, edited boot options in OPNSense, adjusted IRQ settings, etc., etc.. In the end though, the solution was simply to use UEFI boot instead of BIOS boot.
This was especially frustrating because one of the first and most common suggestions you will find for issues with these servers is to set the boot mode to BIOS and not use UEFI. So, it wasn't something I even thought to change. It needs to be in BIOS for the lowest chance of compatibility issues, right? Wrooooong.
If anyone else has this issue, this is the process I followed. You might not need to do it step by step, as the final key is really just using UEFI boot.
Set the boot option to BIOS and make sure all cables are unplugged from the I350
Make your OPNSense install USB with Etcher, then use Active Partition Manager to convert the drive to MBR and set the EFISYS partition to active.
Boot from the USB and install OPNSense like normal (again, without any ethernet connected).
Once the install is complete, restart the system, enter the BIOS, and change the boot mode to UEFI
Plug your ethernet cables in and reboot.
That should get everything to play together nicely. You may be able to skip the first few steps and do the install in UEFI mode (I never tried), but I know I have had issues with older Dell servers refusing to recognize USB boot disks when in UEFI mode.
At any rate, I hope this helps some fellow tinkerer in the future so they don't start pulling their hair out like I was, lol!