I check youtube for replacement videos for battery and screen

You already lost me there. My times of looking up video guides on how to repair my phone are over, I can replace the battery with my hands and the screen is mounted with 8 screws. OEM parts can be ordered directly from Fairphone and arrive within 2 days.

all replacements done with chinese parts, ultra-cheap and locally available.

It is incredibly difficult to get OEM parts for many phones, you never know if it is a good part until you actually have it in your hand. In some cases it doesn't matter but I have never seen a cheap third party screen which comes close to the original.

I figure I’m doing everyone a service by repurposing a discarded 5 year old phone.

You are, the best phone for the environment is an old phone.