There are a couple of ways to get modbus working with a vfd. I have been using vfdmod which seems to work well
https://github.com/aekhv/vfdmod/blob/master/main.cpp
I use a BD600 VFD from Conan motor (in .AU) and there is a complete config for it available on the linuxcnc forum which I used.
yes an encoder would be helpful. Using a Mesa 7i76 with your 7i92 and removing the individual drives from the G540 is probably the best way to achieve this.
I bought an optical encoder from Masso but it has 10 volt output so I'm not sure if the 7i76 would work with it. I am using ethercat so don't yet have an encoder input But I got good results cutting threads by thread milling withot the encoder.