I have experience in developing custom plugins for Flex with their React plugin system. I can get the job done in 4 working days, This is the flow I'm considering.
First milestone is to integrate all the necessary UI components for the plugin, after that it's an open road to integrate the logic. Second milestone will be mostly testing and installing the plugins on your Twilio Flex instance.
I can also scale the solution, I can create a modular component system that you can use to easily override any other actions you need for your Flex integration. It can be as easy as coding your logic and just hitting deploy without any React chore in between.
Regards, Amin