first opnsense router build...any compatibility issues?

[cookiemonster]

seems like your problem is the lack of support from freebsd for this card - see https://www.reddit.com/r/PFSENSE/comments/cdeq47/hp_nc375t_not_working_on_pfsense_box/ as just one example. Maybe a search for "freebsd NC375T" is more fruitful for you. That's an old link.
You'd want a cheap second hand card from ebay or similar but based on an Intel chip.