I think you have the solution very close. I´m sure It is simply a matter of "fine-tuning" the Livolo's power circuitry so that they can provide enough energy (much more than designed) for the extra circuitry, something for which they are not prepared at all, but should not be too difficult to achieve if necessary. Considerations.
1.- The power circuit depends on the "consumption" that can be achieved over the load connected to the switch, so it is necessary to ensure that the load has enough "power draw" to allow pass through at least 100-150mA from AC which should be achieved with a load about 40/60W.
It is also something that can be compensated by adding a bypass capacitor (parallel to the load) of between 100 and 470 nf (of type X2 and 250V better 400 V to go safer), but all this is matter the try and error to achieve the target.
2.- It must be able to avoid the Livolo power circuit so that additional power can be consumed without being affected by the circuit status of the switch, either in standby or active mode. In theory that can be done through the bridges @DJONvl and @Tigroenot indicates in the post you mentions https://forum.mysensors.org/topic/2775/livolo-glass-panel-touch-light-wall-switch-Arduino-433mhz/63 , and whit that should be possible to have the necessary power and several of them seems have confirmed that it is possible.
Perhaps the problem arises to be able to correctly identify which are the correct modifications to be made in each Livolo switch since there are different types of circuits according to the world region to which they are addressed and different revisions of printed circuit so it is critical and not very obvious as Identify the jumpers and connections to be modified and how the power bypass in each case must be performed in order to extract the current to feed the additional circuit.
I think the latter is the most complicated and difficult to perform correctly, but nothing compared to all the work you have already done, so I'm sure you've almost got it.
If I could help you, I suggest some posts (unfortunately only spoken in Russian ;-), o. c. you need use google translator to try to understand) where you will see in the lower page intersting people discussions that are given some patterns and ideas to modify the circuit and that seems have managed to extract required current to power additional circuits.
I also indicated the analysis (I think original) of a person who performed reverse engineering to extract the schematic circuit from the assembled Livolo switch and where it discusses some aspects of its operation that can help.
Please do not be discouraged and persevere a little more than you already have it!