I did not watch the full video yet but I must agree with the title. You should buy a used camera body and get only what you need based on your needs (photo/video, 4k, slo-mo, low light, brand preference).
Been using, the GH4, which is a little outdated but there are some really great MFT prime lenses out there.
When possible I always buy used gear. Works the same and often much cheaper. The only time you should upgrade the camera is when your current one no longer works or no longer meets your needs. Other than that invest in the lens 100% of the time.
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