I recently published my first product on the marketplace.
What I was wondering is, after publishing it, can I delete the link I used on Google Drive to upload the project?
Then another thing, if I delete the project on the computer, in case of changes, can I easily re-download it to be able to work on it again?
The link you provided is only used during the approval process, the project files are then hosted on Epic servers. So if your product was approved deleting the files shouldn’t be a problem.
Regarding your second question, you can download the product after it is published like the end-user would, so add/create project from the Epic Games Launcher. Regardless, I would advise you to never completely delete your project files from your computer, or at least to always keep a backup of your client/marketplace projects on an external hard drive.