The only destinations that show up on Google takeout are: Flickr, Microsoft OneDrive, SmugMug. There is no iCloud option!

I have a windows PC. By downloading them to Photos library, you mean I have to install iCloud app on my pc and download them in to the Photos folder within?

I have around 7000 pics and ~2000 vids. So if i want to download everything I might need to get a new HDD with enough space in order to do so.

I was planning on doing this: Transfer a copy of your photos and videos to iCloud Photos - Apple Support

but i got stuck at step 3. - Choose "Apple - iCloud Photos" as the destination to transfer and sign in with your Apple ID.

This option simply doesn't show. And I think it could save me a lot of time/energy/space(money) if I could make this work.

I don't use Windows so I can not comment how to best use Photos library or some other 3rd party option.

In my location the Google takeout link below says transferring Google Photos to iCloud Photos "coming soon" so it is not available here. I had not seen that option so thanks for sharing the link.

https://takeout.google.com/takeout/transfer/custom/photos

That is one to ask Google, as they set up that Take Out service.

Login to Google Takeout, "clear all" selections, make a desired selection (i.e. just Google Photos and all/selected albums), download it as a .zip file (might take a while if there are lots of files), and unzip it to reveal the images and movies. Then download them to Photos library.

https://takeout.google.com/

That downloads the images and movies with the same metadata as when you uploaded them to Google Photos. Beware that Google Photos "space saver" option converts movies to VP9 codec that is incompatible with macOS, though (I have not tested if that applies also to takeout).

If you have done any metadata edits inside Google Photos (date, location, description, favorite) and want to preserve those edits, then you have to use GraphicConverter or exiftool to copy that info from the .json sidecar files to the images and movies. Google Photos has some glitches and bugs that might complicate the .json copy so ask if something does not work as expected.