have you tried testing it with iperf3 yet?
How about the hdparm to the harddrive to ensure that it isn’t a chokepoint
What is the drive partitioned as NTFS or EXT3/4?
lsusb -t
To see if the drive is actually connecting to the RPi board at USB3 or or USB 2 speeds
Jeff Geerling did some testing
Samba is a cpu heavy sharing service…RPi is a computer…but is also an anemic processor, also the HDD might not be running at full USB speeds…USB2.0 is what theoretical 480Mb/s…so what 60~MB/s
Also…USB storage stuff has been a sticking point for RPi
https://forums.raspberrypi.com/viewtopic.php?t=243962#p1786090