Hi again,
Reserved space and Quotas are different. Reserved space is space that is... reserved, so no other can use it. As such, this space is guaranteed to be available to that dataset. It is a minimum space that this dataset is sure to have.
Quotas are maximums instead of minimum. A dataset will be prevented from using more space than this quotas, so as you said, it will not fill the drive.
From your screenshot, Plex is definitely a big user, with 1.58 TB of data. Your Windows share is next, with 610 Gb, or 0.61 TB. Just add these 2 and you have the 2.19 TB of space used over the pool. Everything else, including the jails, is not significant.
Either you clean up your Plex, your Windows share or go get more storage if you need everything you have as of now.
Adding more drives will allow for the creation of new vDevs or replacing your actual drives one at a time with bigger ones and re-silvering between each change will auto-expand your pool once the last drive will be activated.