The documentation doesn’t specifically state that the Dashboard needs to be at the latest version to apply a firmware update:
Also, I’m not a Mac user, so I’m not sure why you cannot install the latest Dashboard version with High Sierra.
The Dashboard and firmware will never be “matched” with the firmware. The latest Dashboard version for Mac is 3.6.1 and the latest firmware is 4.2.1.
Further to your specific question, though. I can say with 100% certainty that you can upgrade your firmware to 4.2.1 using a Dashboard version of 3.5.1. Here’s why:
Firmware 4.2.1 was released on April 9, 2020:
Dashboard 3.5.1 was released in October 2020, therefore it would already have supported firmware 4.2.1 at the time:
I hope that helps!