The most economic and efficient way of storing around 20 TB of data is to buy an HDD and store it on that. Your best options are probably either a Seagate Exos X20 or a Western Digital Elements. Enterprise SSDs are possible options,but far more expensive than an HDD. You especially do not want to buy an Enterprise SSD, keep it offline and power it on occasionally.

Is NAS the perfect solution? Or should I use internal HDDs as the main storage and daily use? Thanks,

Depends a lot on your particular use-case. For most people OneDrive is a pretty good place to store personal files like documents.

No. The storage industry is not a scam. HDDs are incredibly complex and delicate mechanical devices, that fail, despite manufacturers putting in enormous efforts to make them as reliable as possible.Portable HDDs are even more vulnerable, and you shouldn’t consider them long term investments. You should rconsider using portable HDDs as your primary storage, and set up a proper backup strategy.