NAS getting full…

Not sure if I should post this here or in the Plex forum…. I have a DS1821+ with eight Seagate Exos 18TB drives; six for data and two for redundancy. Using the box mostly as a Plex server, and I am running out of space.

I understand that there is a pool limit of 108TB, which I suppose I have reached with my current configuration. If I now start replacing drives with a higher capacity, would I have to use the new space for a different pool? But presumably I’d just create a new folder in the new pool and add it to an existing library in my Plex setup? Or would I have to install a second Plex server in the new drive pool space to access any new libraries?

Should have bought bigger drives when I set up my box two years ago, but thought that’d be enough space (hah!). Can anyone tell me what the biggest supported Exos drives are that I can use (was there an issue that some drives were using too much power and wouldn’t spin up, or something like that)? And/or which drive sizes currently would be the best bang for the buck?

Thank you.

EDIT: volume limit, not pool limit. Words matter.