Ceiling lamp offset.

I will refresh the old so as not to start a new thread.
I need to move the ceiling lamp about 15-20 cm. Unfortunately, the direction of the shift is opposite to that from which the cable is coming. In the present place, the cable is so long that I would like to make a 20 cm furrow in the plaster and let the protruding cables run through it. The problem is that the outer cover is removed at the point where the cable exits the ceiling. There are three long veins, each with its own undamaged insulation. I was thinking about additionally securing them together with insulating tape (possibly first self-vulcanizing tape), inserting them into the furrow, then gypsum mortar and finishing coat.
Will this solution be acceptable? Any other suggestions?

Moderated By krzysiek7:

I separated from: https://www.elektroda.pl/rtvforum/topic2672889.html

There are fans of the use of extension cords on the forum.
So be careful, because they may come up with an idea to end what is sticking out of the ceiling with a socket and power the luminaire with an extension cord.
I suggest that you do not take such an "idea" seriously.

Maybe you have installation in pipes and single-core cables?

kkas12 wrote:

Maybe you have installation in pipes and single-core cables?

No, the cable has three wires in one sheath and is directly in the plaster. Those veins sticking out are of sufficient length. I am looking for a solution how to wisely release them "further" in the ceiling.

Use a heat shrink tubing, but it must also cover a piece of insulation (maybe 2-3 cm) of the wire.

kkas12 wrote:

Use a heat shrink tubing, but it must also cover a piece of insulation (maybe 2-3 cm) of the wire.

OK, so I will throw all the wires into one tube, put a bit on the end of the cover, "heat" it up in the groove.
Thanks.