Execution of Gable Wall Interior

Hello,
We bought a house from the 70s and are carrying out a complete renovation there.
We have completely opened up the ceiling in the attic.
We would like to close off the now visible gable along the two beams upwards with drywall (see pictures).
What comes between the tiles and the drywall? We have a 60mm insulation on the outside which we plan to extend by another 100mm in 2 years.

An acquaintance said that I could brace the gable wall (between the first and second row of tiles) with a 100mm beam.

Is that really sensible or what would you put in between?

Thank you very much for your answers.

Does no one have any advice? I see 4 options here:

1. Leave everything as it is and simply cover it with drywall.
2. Rebuild the second row of bricks.
3. Insulate the space between the bricks and the drywall on the inside, although there is already external insulation.
4. Brace the gable wall with a 100mm beam.