ASIC.to has been reliable site in the past for decent alternative firmware for S9 and S17 series. I have used on both of those without any issues in the past.
I currently use MSK miner (mskminer.com) on my S17 Pros. They are both comparable and I just like MSK a little better.
They both have Developer fee's built in of about 2.8%.

The ASIC.to review on YouTube stated the S19j was 2%, but I can find any more details yet.
I am also considering trying the S19J firmware from ASIC.to
It appears their are no links on their site for the S19j firmware at the moment, but very active chat on the Telegram Channel.
I'm reviewing the chat now and may try it or may not.
My only worry is my units are under Bitmain Warranty, and Bitmain will deny warranty if they detect modified firmware having been installed.