But i dont understand: ,,Keep in mind that pruning still downloads all those 300GB (minus what you already have), hence it's still slow.,, - what is purpose of pruning then?
Bitcoin core wallet basically trusts nobody.
This means that your wallet will still download the whole blockchain and verify it, but will save to HDD only the data it decides that you'll need for your wallet and transactions. (I am not sure it's an accurate definition, but it should give you an image).
And that 550 tells the size it should use for the saved data (550 is the minimum, it's 550 MB).
I only have about 200 gb of space or can I use external hdd?
Well, go to the data directory, I gave you the link about it in a previous post.
And for the Blocks folder right-click, properties and see what's it current size. Make the difference and you'll find out more or less the size you need left. (Keep in mind that the 300GB I told you is a rough estimation, I don't know the current size, I don't have my core wallet synced for some while too).
Since your wallet was synced in 17.7.2020, the remaining 200GB from your HDD should easily be enough.