Thunderbolt 2 can be converted to USB, but I think they don’t have 10 Gbits USB 3.1 gen 2 from TB2 adapters, so it’s no different than the built in USB 3.0 ports.
Just an FYI you don’t need adapters when going from USB-C to any USB host, you just need the right cable, which for a lot of SSDs they include USB-A to C and C to C cables so you could connect to any USB computer.
However, there are SSDs that are Thunderbolt only. Maybe your best bet for the older Mac is on older Thunderbolt 2 SSDs, which can be used with the Thunderbolt 2 Macs as well as Thunderbolt 3 Macs through the Apple TB2 to TB3 adapter. You can also go the other way, as the adapter is bidirectional, as in a TB3 SSD with the TB2 Macs. Do not, however, make the mistake of thinking that adapter should work with USB devices, as it is Thunderbolt protocol only. People have bought the Apple TB3 to TB2 adapter and thought it should work with DisplayPort monitors, or USB-C devices, or other things that aren’t strictly Thunderbolt, and then left bad reviews saying how it doesn’t work where it wasn’t designed to work.
Probably even more confusingly there are a new generation of Thunderbolt 3 devices that can be used with any USB host. They use what’s called Titan Ridge controllers, which support USB and Thunderbolt protocols, so those are fully universal and work with virtually any computer with USB or Thunderbolt. Those are probably gonna be the most expensive drives on the market.
If you’re fine with 5 Gbits speeds though, you should find any number of USB-C SSDs like Samsung T7. It includes the USB-C to A cable you need for the 2014 and you could use either USB-C to C for the M1 Mac Mini for 10 Gbits speeds, or C to A cable for 5 Gbits speeds because Apple’s USB A ports are only 5 Gbits (there are 10 Gbits USB A ports but Apple has never included one of those on any product of theirs).
If you still wanted the fastest for both then the is Titan Ridge and would work with Thunderbolt on both computers (through the Apple adapter for the 2014). In addition to also working with USB-only hosts, like most pre-Thunderbolt 3 Windows machines. But while it’s about twice the speed, it’s also twice the price over the T7.