Shimano R785 Di2 compatibility

I was one of those early adopters who went for Shimano R785 Di2 electronic gears and disc brakes. The shifters and disc brakes still work beautifully, but I want to change derailleurs and chainset. My question is, how compatible is the new (Ultegra) Di2 kit with the old R785 shifters?
Can I use Ultegra Di2 derailleurs with the old levers? Or GRX for that matter?
Could I switch to an internal battery (R785 used an external battery)?
Any thoughts much appreciated.

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