Replacing, instead of repairing, DNA
It seems very feasible to synthesize human DNA (please inform my
  ignorance, if I'm wrong. lol.) and it looks very promising after
  reading this:
  https://www.wired.com/story/live-forever-synthetic-human-genome/.


It is certainly possible to synthesize human DNA, but it's another thing to create a functional chromosome. In fact, in the article you link to, they are discussing a research effort to synthesize a human Y chromosome. As far as I can tell they haven't actually done this yet, they are just proposing to attempt it. George Church is a highly regarded genomicist, but the article is very sloppy about distinguishing what they think they can do from what they've actually done. There has been work with human artificial chromosomes (HAC), but these have been special purpose constructs that successfully replicate and divide in cells, but don't duplicate an entire natural chromosome.

Synthesizing a human genome would just be the start of the therapeutic process. You then face the question of how to place the synthesized genome in the cells of the recipient. Current approaches to gene therapy involve inserting a single "repaired" gene into a virus, and then infecting the relevant tissue with that virus. This doesn't insert the gene into all cells of the body, just the ones infected with the virus. For some simple genetic diseases this is enough to relieve the symptoms of disease. You can't insert an entire human genome into a virus though.

It's also been proposed to harvest stem cells from the recipient, modify them with the repaired gene, and re-insert them in the diseased tissue. Again this would affect only a portion of a single tissue, not all the cells of the body. It's also one thing to insert a short sequence into a stem cell, but nobody knows if you could swap out a stem cell's entire genome and still maintain it as a stem cell. Also, this would only work for tissues that are still actively dividing. Many important tissues (in the brain for example) have stopped dividing or divide only slowly in adults.

Even gene therapy involving single genes has been approved in only a handful of diseases. It's sill almost entirely experimental. What you are suggesting is still in the realm of wild speculation. Speculation can be inspiring and motivates research programs, but don't starting counting chickens yet.