Currently I'm seeing approximately 110-130 MB/s over the 10Gbps connection, which I would have expected to be faster given the history and the following data.
with the MTU set to 9000 on both machines
Make sure any switches/bridges in the middle also have an MTU of 9000.
The network clearly transferred 10Gb/s from your iperf test so it's likely not a network issue.
The host was writing about 7Gb/s with your local write test. The file copy was close to 50% that given it has to read and write to and from the same dataset, so that could make total sense too.
As a final test, I SCP'd
SCP has encryption overhead, forget that test.
At this point some suggestions:
Make sure the host can actually read whatever file it's reading at 10Gb/s too. If it's just some standard consumer SSD chances are it can't actually read out at 10Gbe anyway. We already know the network can do it with the iperf test you did.
Check the SMB configuration. There are optimizations to be done for 10Gbe to be useful on Samba servers. Very serious and worth googling some recommended config options/guides.