Says on the product page that that Beelink supports a maximum 2TB SSD, so I wouldn’t go buying another unless you’re sure it can.
As others have said, no USB drives as I’ve had a failure with one too hence me now having a NAS. I know you said you’re allergic to spending money but saving up for one was the best thing I’ve bought for a while.
Nah.
USB is slow, and will kill the drive if used 24/7. I know it’s hard to expend too much money on a dedicated box for storage but long term you’ll thank yourself for investing.
3TB of data I lost and paying to recover that data? Dammit, hindsight is a bitch