Plasterboard Work
It is often possible to run a new cable between a fixture and a switch without tearing up large sections of drywall.
If you can visualize the wire going up the wall from the switch and then from the corner of the wall/ceiling to the fixture, you may be able to snake a new wire.
Cut a small hole in the ceiling in in the plasterboard immediately above the switch.
The issue will be whether the hole in the ceiling allows you a straight run to the fixture box (along or between a ceiling rafters). If not, you nay need to cut more small holes where the cable would have to cross a joist/rafter.
If it is a clear run, you may be able to run a cable from the ceiling hole to the fixture box. Then you can cut a small hole in the wall immediately below the first hole.
You also need to be able to drill through or notch the framing (without seriously compromising a load bearing wall) to allow a cable to pass through. If this is feasible, you can drop a cable down to the switch box, thread the other end through the hole or notch in the framing, and extend it to the fixture box. If you are notching or if you are drilling at less than 1.5 inches in, you need to put a special metal plate over that spot to protect the cable.
Later you can patch the small cutouts, tape over and paint.
Wiring
You can continue to use the existing cable to power one circuit and use a 14-2 cable to provide the hot leads for the other two circuits (they will all use the neutral already on the circuit. Since both wires in the new cable will be hot leads for a circuit, the white wire needs to be marked at the switch and fixture to show that it is hot.
You can find three switch units that fit a single gang box, or expand the switch box to double gang.
Google Home is nice, but you can't control primary room light exclusively with it.
The building codes absolutely require that some lighting in each room be controlled by plain wall switches in standard locations. The locations of the mandatory switches are defined by Code. The brightness level is defined by Code, though in some living spaces it is allowed to switch a receptacle and the citizen is on his own recognizance to plug in a workable light.
As long as there is a light switch in the proper location, and it operates some light in the normal way, the Code requirement is satisfied. You can have as many other lights as you please, control them any way you please, and you need never use the statutory light switch.
Lack of legal lighting could endanger your occupancy permit and thus, your mortgage. What's more, if an accident is caused by someone being unable to turn on the light, the insurer will cite the defect as reason to refuse coverage. Your mortgage lender will have a problem with that too.
On top of this, that Sonoff junk is a direct import from China and uses this loophole to bypass all our safety regulations, that's why it's cheaper. As such, it is not approved by a Nationally Recognized Testing Laboratory (NEC 110.2 on the very first page) so there goes your fire insurance. And again if the inspector catches this in your home, he'll make you tear it all out. Rules are eased somewhat for smart devices which are not made part of the house wiring and are merely plugged in.
Best Answer
I would hesitate to give a definitive answer to this because it's obvious in your circuit that when S1 is ON and S2 is OFF then you are connecting mains to the output of Sonoff 1 with no connection to it's input.
However, I have checked my own Sonoff (Basic R2) and the input line is switched to the output by a mechanical relay with no other connection to the output so I can see no reason why this wouldn't be perfectly safe. Worth checking yours is the same though as other versions may be different.