Through a ton of research and some luck, I have finally stumbled across the solution to this issue with FCPX SMB performance in macOS Catalina. The issue seems to be that macOS Catalina default system parameters for networking are not tuned for high demand network performance. That being said, it seems that at some point in time as far back as OS X Mountain Lion some engineers at Apple created a feature called “Server Performance Mode”. Imagine that, a feature in macOS literally named after creating a network performance mode. This feature is documented on Apple.com
Server Performance Mode
https://support.apple.com/en-us/HT202528
The description reads:
Performance mode changes the system parameters of your Mac. These changes take better advantage of your hardware for demanding server applications. A Mac that needs to run high-performance services can turn on performance mode to dedicate additional system resources for server applications.
Solution:
1 - First check to see if server performance mode is enabled on your machine using this Terminal command. You should see the command return serverperfmode=1 if it is enabled.
nvram boot-args
2 - If you do not see serverperfmode=1 returned, enter this following line of code to enable it. (I recommend rebooting your system afterwards)
sudo nvram boot-args="serverperfmode=1 $(nvram boot-args 2>/dev/null | cut -f 2-)"
On my brand new Mac Pro (2019), not only did this fix issues with Final Cut Pro X freezing and disconnecting SMB network volume shares, it increased my throughput performance from 30%-50%. My machine almost reaches the theoretical max throughput of 10GbE. I have also tested this on older machines running Mojave and they also gained performance improvements. I have also tested older "trash can" Mac Pro's upgraded to Catalina that were previously also crashing and disconnecting SMB shares and they too are now working. I can't promise that this will fix everybody's issues, but I can tell you that there is a good chance it will help your overall network performance especially if you are using a high demanding network application. Note: I have not tested this, but if you ever execute an NVRAM reset this option would presumably disappear and need to be re-enabled.
If at any time you want to turn off performance mode use this line of code. (I recommend rebooting your system afterwards)
sudo nvram boot-args="$(nvram boot-args 2>/dev/null | sed -e $'s/boot-args\t//;s/serverperfmode=1//')"
Final notes, I have been talking through Apple directly throughout troubleshooting this issue. They at no point ever provided me any help or clues to solve this issue. Apple's official stance is that this is clearly a "third-party" hardware issue that is simply not compatible with macOS Catalina since this issue is not as pronounced in Mojave. They would not even entertain any notion that anything related to macOS Catalina network configurations could cause or help network performance issues. To say their support for this issue has been underwhelming would be a gross understatement. It's also mind-blowing that the FCPX team and Apple engineers are so unfamiliar with anything related to network performance in macOS that they could not have provided this as a possible solution or even suggestion. I could say a lot more as it has been an incredibly frustrating experience as an Apple customer, but to that point I wanted to share this with the world and the broader FCPX community that there is a way to improve your network performance in macOS Catalina and it doesn't have to depend on third-party support like QNAP, Synology, Lumaforge, and SNS. I am sure those folks can fine tune the performance even more, but at least this feature engineered by Apple exists and there is a good chance it will help your situation.
Enjoy!
PS. Apple engineers if you guys ever read this. Please consider making these network configurations more transparent and easily accessible to the user. I don't know why macOS couldn't have a network configuration option existing as a button or check box somewhere that would enable Server Performance Mode. Also, consider testing apps like FCPX more thoroughly in common shared storage environments and consider all performance options including macOS.