I’m assuming if you want 2.5GigE or 10GigE, you have large data to move around. Apple includes 10GigE option on the Mac Mini because many of the use cases that people buy Mac hardware (and macOS) for is video editing of 4K or 8K footage. Before the Mini had 10GigE options, there were Thunderbolt adaptors to direct attach storage arrays (think an external array of 5 to 10 disks exclusively attached to one computer) for the speed.
This is the lowest product that supports the upgrade card, it has 2 x 1GigE ports on the back:
https://www.synology.com/en-uk/products/DS723+
The network upgrade card is here:
https://www.synology.com/en-uk/products/E10G22-T1-Mini - US import around £110 on Amazon, EU import around £130 on amazon. May be cheaper elsewhere (e.g. Broadbandbuyer). Stock levels vary.
I’ve not bought the card yet as I’ve not bought a fast enough switch, and my WAN may get to the 900/900 speeds if FTTP arrives this year…
23 years of broadband connectivity since 1999 trial - Live BQM