Don't consort with fools
There is a spectrum in intelligence. The stupidest people buy commercial off-the-shelf solutions because they can't design a system. The midrange intelligence can design a system, so they do because it makes them feel like the smartest. Problem: becoming absolutely responsible for the system working, because no contractor will touch it with a 3041mm (10 foot) pole for fear of liability. Being wise to that problem, the smartest people buy commercial off-the-shelf solutions because they need it to actually work and because if it isn't working and they can't be bothered dealing with it, they want to Just Call Someone and it's fixed.
In that light, the "middle intelligence" solution is actually the dumbest.
Seriously. Raspberry Pi, are you kidding me?
So, don't you touch this project with a 3041mm (10 foot) pole. This person is going to fail badly, and when the rest of the greater family is freezing their tails off and turns desperately to you, you want to say
I recommend a commercial solution.
Now I notice you're concerned with heating this time of year (so not Australia or South Africa), and you don't seem American or Canadian, so that leaves folks in a bloody mess for gas supply. At least you have a high efficiency condensing boiler. Unfortunately this requires new thinking if you actually want to cash in on that efficiency.
There can be a place for homebrew automation, but it should act like a "trim tab" on an airplane - capable of fine tunes but not capable of breaking the system if due to a malfunction it went "hard over" in the worst possible direction. That's my opinion. A bad design would be one where the automation needs to hold a valve open for there to be any heat at all.
BANG-BANG is dead. Long live condensing!
The old thinking was that you had a thermostat that waited until it was 1 degree below setpoint, then BANG! It turned on the furnace, which ran until 1 degree above setpoint, then BANG! It turned off the furnace. Heat level is controlled by duty cycle - on a warm day it runs 5% of the time, during the worst case conditions it runs 90% of the time. This Bang-Bang "thinking" is a disease that blocks understanding of your system.
Because - you have a condensing unit. Your unit takes methane or propane (already a gas) and mixes it with oxygen from the air (already a gas) and makes CO2 and water vapor (already a gas). We then have an exhaust-water heat exchanger and it's so-and-so efficient. But a funny thing happens. Remember that water vapor? To boil a kilogram of water takes 630 watt-hours or 2150 BTU to overcome latent heat of vaporization. We didn't pay for that, the components were already vapor - but if our heat exchanger is good enough, we can take that energy. And that's what a condensing unit does that makes it so darned efficient. This means either an absolutely huge heat exchanger, or a normal-sized one run at much lower power/flame. The lower the flame, the more efficient the unit runs.
So to score that efficiency, "BANG-BANG" must be thrown away. The new paradigm is the system runs continuously at the lowest power setting which will heat the building, adjusting the power setting to suit the demand. That is why your unit has an outdoor temperature sensor. It can adjust its power setting to correlate to the amount of heat your house needs to "hold even" with losses through insulation, which are proportional to temperature difference.
And this is where your buddy's plan flames out. It has solenoid valves or pumps that it wants to turn on BANG or turn off BANG to suit resident need. That's what the smart sockets are for. That is totally incompatible with the design concept of this condensing boiler.
What you really need is "trim" adjustments on each unit, or each radiator, to slightly increase or decrease their flow resistance, so they get more or less flow relative to the other units. It might be viable to BANG-interrupt 1 unit at a time, so sometimes 3 are running instead of 4, but that's as far as it can be pushed.
Of course, the appeal of BANG-BANG is that when you make a temperature change, the system goes BANG! full-on, or BANG! full-off, until it reaches the newly commanded set point. Your demanding customers grew up expecting that kind of BANG-BANG "fast as I can get there" responsiveness, and aren't going to take well to the slow and easy transitions made by fine adjustments. This also "takes some getting used to" for Americans accustomed to time-of-day thermostat adjustments.
Given a group of irresponsible people who don't get it, just forget hydronic and go mini-split
Or keep the hydronic in the background for baseline "get it to 15C/60F" and then have them run their individual mini-split heat pumps to fine-tune the temperature to the degree they'd like. That would help stir the air anyway; a major factor in comfort is the heat separating with the hot air in the ceiling and the cold air at the floor.
Bonus points: the mini-splits provide air conditioning. And and that fits hand in glove: a mini-split right-sized for supplemental heating will be right-sized for air conditioning in most places.
Splitting bills on a large house makes no sense.
You have shared walls, and those walls are not a source of heat loss. Occupants with a lot of shared walls have easier heating bills than people with a huge amount of outside-wall or roof exposure. And that's completely unfair, because it has no reflection whatsoever on the square footage (meterage?) of the dwelling. The people on the top floor just get mauled, because they are losing heat through the walls and the roof. Whereas the guy with a middle unit with other occupants above, below and to the side is a Free Rider, getting most of their walls heated for free by other occupants, and only has loss out their relatively small outside wall surface.
The only fair way to split the heating bill, then, is by total square footage (floor space in their private zone) with some proportional division of commons spaces, perhaps in the same ratio.
As said, if some want the thermostat set higher than others, they can use supplemental heating of their own - cheap resistive heaters (install baseboards, don't let them use unsafe plug-in heater since a house fire affects everyone)... or as I proposed they can buy a mini-split heat pump.
Insulation comes first
Once you realize that leakage through exterior walls is your actual enemy and not each other, suddenly insulation rises to the top of the shared interests. Insulation is cheaper than fuel because you only have to buy it once, and not from "unfriendly nations". If it wasn't for your communal situation, everyone would agree insulation is Job One.