I don't know anything about Di2 but this is part of the reason that I said that using conventional derailleurs is a bad idea. It would be better off in the end to just build your own derailleurs that wouldn't use a return spring at all.

The only reason you need a particular derailleur nowadays, is because it is made to work with some particular mechanical shift indexing system, that is built to work with some arrangement of chainrings and a rear cluster with a set number of cogs. With a totally-digital system, there would not be any mechanical indexing--you would have to program that in yourself, and then the "index" would just be a series of settings in the computer's memory. So it could be made to work with any different arrangement of chainrings and rear cogs you wanted, perfectly.
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For this reason alone I doubt normal drive train companies would ever do this, as they'd sell a lot less different sets of parts.

If the gear selector was digital, you could have it do a few neat things:
...you could have it access ALL the gears in ascending/descending order, and you could tell it to skip gears that are duplicates or that cross-chain badly. There would just be a [shift up] button and a [shift down] button and the computer would know what combination to use for the next-higher and next-lower gear in the programmed setup, even if it involved switching both the rear and front derailleurs.

...if you had a pedal-sensor, you could make the electronic shifter NOT shift when you are putting pressure on the pedals. It could automatically shift in-between pedal strokes, every time. A lot less crunching that way...

...if you had a bike-speed sensor, you could program the shifter to ignore shift-button presses below a certain (riding) speed, so you'd never accidentally shift it while standing still and have the gear-crunching issue when trying to get started.

...also if there was a speed-sensor, you could set the shifter to work differently at different speeds--such as, at moderate speeds it could "over-shift" a bit more, and then back down to the target chainring or cog (just like what you do manually with friction gears) so you'd get faster shifts that way.

An Arduino Pro Mini only costs $1.50 for Chinese clones, and measures about 5/8" x 7/8".
I dislike using RC servos however.
Stepper motors are more precise (they don't accumulate positional errors), they are built more robust and they don't require a constant signal just to hold a stationary position.
If I was doing this, I would start out trying to use a nema-8 motor, and if that's too weak and slow then try using a nema-11.