You distribute it, so you are required to provide the source code.
That's how the GPLv3 license works.
Actually, I am in the same position as you but what am I trying to say is that 99% of this industry is not respecting the License.
Let's make it more clear,
Bitmain, Whatsminer, Innosilicon, and many more Even Goldshell that TSUB uses, which uses BFGMiner that also is violating the GPLv3 license.
So can we say 90% of the mining industry is currently violating Licenses? this means that 90% of your pool users are using devices and firmware that violate the GPLv3 license.
As Mineitor team we have three goals
1.Support any model and brand of miners which includes custom firmwares. so we can reach as many users.
2.Provide the best possible Customer Service.
3.Provide the best tools possible for monitoring and management.
As result, we can't lose the part of customers that is willing to use custom firmware. so we launched our partnership program which includes Vnish and in the future, there will be more.
Let's be honest with each other. You can't prevent them from using your pool. If you do you then you will have to shutdown your service.
I looked at your website. You only prevent Custom Firmware. If you care, In the first place you need to stop using miners and giving service to them. Even
firmware provided by the miner manufacturer