iMac using Mac Mini for storage

Target Disk Mode used to be a good way of getting files off an old Mac's internal drive. (I wouldn't recommend using an old Mac in place of a new external drive.)

But now, if either computer has Big Sur or later installed, TDM only works with a Thunderbolt connection – and that would rule out a connection to some old Macs, while requiring a $50 adapter + $30 TB 1/2 cable in order to make the connection to others.

Transfer files between two Mac computers using target disk mode - Apple Support

So even for data transfers, it may be better to clone the old Mac's drive to an external USB drive and then move that external USB drive to the new machine.

+1 While some year models (2010-2012) with an HDD to SSD upgrade, make decent media servers. Using the Mac mini it is not practical or efficient, as just using a regular external drive with the new iMac.

Save your self the hassles and power and just get an external drive.