What is the expected durability of a Mac mini (2020) flash storage?
I've had my 2018 Mac mini with 512 GB since November 2018 (around 580 days). According to DriveDx, the on-board SSD has been powered on for 28 days, despite the fact that the Mac has been switched on almost permanently.
The drive has written a total of 17,1 TB of data, according to the SMART data from DriveDx. I've heard 300-600 TBW bandied about as a typical SSD lifespan. (300 for the blade on a 2014 MBP; 600 for a 2018 Samsung EVO NVMe.) In which case, I've used about 0.06% - 0.03% of the drive's writes.
So it should last me 26-52 years. You may be able to extrapolate a result based on your own usage.
If you are expecting to write 8 times the disk's size every month, then using a larger external drive, which can be replaced more cheaply if necessary, may be a safer/better option.