Thanks for everyone's help, I believe the issue is the benchmark software I used. For some reason, DiskMark 2.1 reports the sequential read speed at ~1,500 MB/s but other benchmark tools (Black Magic, Aja, AmorphousDiskMark) report sequential read speeds around 3,000 MB/s and writes just under that. Not sure why DiskMark 2.1 is so different than the others.
So, with that said, I don't believe I used that Synology card in slot 2 when I tested it, so that is probably why I didn't see any improvements. If that's the case, I installed in slot 3 or 4, which is 4x and the card is 8x, so that would certainly make the difference. If I get a chance to retest, I'll post the results here.
Thanks again!