You mentioned that cloning is possible. Ethically its really impossible if we're going to do it for a lot of people.

That is, until very recently. Theres actually a very new procedure where a person in need of a heart transplant can first transplant their own DNA into the pig's living heart using CRISPR, and then transplant the gene edited pig heart (or any organ likely) into their own body, and there will actually be a lower risk of organ rejection than in a human to human transplant. Essentially, its an ethical and superior form of organ transplantation that is really just emerging in the past year.

Still, that doesn't change what happens to the brain. I like your question because its a thought ive been stumped on for maybe 5 years now. I have an idea that i'll share for fun mostly. Here it is:

Cellular quantum beam implantation. We implant whole organs using a surgeons hands, scalpels, etc, but what future technology will trace its roots to today's organ transplantation? Cellular quantum beam implantation. The idea is very simple. Cut a hole in the human body using, say, a thin laser beam. Specifically, through the skull and into the brain. The laser beam is really multiple laser beams. The first part of the beam is basically just used as a highly accurate cellular radar. It's possible to map the individual cells in the skin, skull, and brain. Using that information, aim the second part of the laser beam, the high energy cutting mechanism, in key locations that allow for tunneling deep into the brain while producing as little damage as possible. The final component is slowly accelerating individual brain cells extracted from either a clone or gene edited mammal using a large particle accelerator. The living human brain is situated in the particle accelerators path. The tunnel cut into the brain is just large enough for the fresh young brain cell to be decelerated neatly into place, guided by the particle accelerator. A straight path isnt really neccessary, using perhaps a graphene thread/rod you could prod the cell into place from many angles and through many tunnels. (Rotate the brain). The cutting and mapping mechanism wouldnt neccessarily be a photon beam either, such as in a laser or radar, but might instead consist of some other quantum beam only possible in a particle accelerator. Sever the beam, and if the tunnel you cut into the brain was narrow enough, then no neurons should have been severed. The fresh young cell would hopefully grow into the place of the old cell in the same neuronal network. It might seem simpler to edit the genes of brain cells to prevent aging - but that might be impossible.

Anyway, its an idea in the beginning stages. I havent looked into those papers where people mess with brain organoids. But thats where i would start to try and understand how this could work. Im fairly confident the tunneling component of the surgery would work (and im also fairly confident it could be done in a way that no stitches are needed and that patients could have cells implanted at a rate of hundreds per second at least, im just not sure if the human brain would be okay with extracting old cells and having them replaced with young cells. Additionally, even if the brain could survive such a procedure without neurons being severed, how would the mind be affected? Would it be something like a lobotomy?