Shimano has a pretty good compatibility chart.
These things are "incompatible" according to the chart
- 11/12-speed drivetrain, I guess that should be clear, so the rear mech won't deal with a 11-speed cassette and you should run a 12-speed chain which is a tiny bit narrower and optimized for the cassette. Even if would be easy to adjust electronic gearing to narrower 12-speed gaps than a mechanical shifter with physical "clicks", Shimano has no intention to make this compatible and update their firmware, I presume.
- The 11-speed crank and the 12-speed derailleur: There are no specifics given but there is 1 mm chainline difference. That's something that can probably be worked out, at least on a mechanical groupset, it might be that Di2 adjustment is a bit more delicate or shifts not as crisp as on a full Di2 group but I have no practical experience.
- According to the chart, the new Di2 ST-R7170 levers are not compatible to your old BR-R7070 calipers, however, the other way around, ST-R7020 (your current brifters) work with new calipers, so I guess that should be OK and just Shimano keeping things simple in diagram, no real limitation. Same brake hoses, same pads, only their homepage claims 10% more pad clearance, I assume this refers to the previous generation of calipers.
So, the minimum you need for an upgrade is
- STIs (left/right)
- Cassette (will fit your wheel)
- Di2 Front/rear derailleur
- 12-speed chain
- Di2 battery
- (possibly a 12-speed R7100 crank if it turns out to be problematic)
If I just put these on my cart at my usual shop, it is worth about 900€ of components but they have a deal on the full set for under 1200€ which gives you calipers, rotors and matching components.
So, it might be worth considering doing a full upgrade and just sell or re-use the leftover R7000 parts.
The only thing I'm not sure about is availability of a 52/36 crank, the chart shows compatibility of the derailleur (max. 52t) and Shimano's Shimano's product page lists it as an option for FC-R7100 but I haven't found a mid-compact option in the 105 range, neither as an individual component nor as part of a full 105 R7100 groupset, only 50/34 in all crank length options from 160-175 mm.
You would have to work around by using a non-Shimano 12-speed crankset or using a 12-speed Ultegra crank (which has a 52/36 option it its current generation) - or accept the lower gearing, I only use 50-11 twice a year on a big descent.
Note: Last section about chainring sizes was written to the best of my knowledge at the time the answer was given, if Shimano decides to widen its range, kindly ignore it.