old parquet floor - what to do

This floor might be over 50 years old. I have sanded and re-sealed or re-coated 6 years ago, however totally forgot what I have used for the coating as this is my first DIY project. There is a large sliding door in this room facing the sun so the floor colour changed (became lighter) a lot in the area next to the sliding door, the coating seems wore off, the timber colour in the other areas of this room away from the sun seem unchanged . A picture is attached. At the moment I still want to keep the timber floor, so not sure what I should do, appreciate if someone can give me some advices. thanks in advance!
1. Do I need to sand the floor to bare? if not, can i use Peerless All Purpose Stripping Emulsion Floor Stripper, then lightly sanded afterwards?
2. what coat/sealer should I use? this room will have high traffic (middle of this room) , and high sun exposure as well
3. The size for this room is around 20 square meters
4. I am a beginner of woodwork

Get a professional floor sander to look at it. It’s a professional job you want done, get someone who knows to do it.