First of all, there is no such thing as a Mac mini 2020. You have a Mac mini 2018; the only thing Apple changed this year was the baseline SSD storage amount. Everything else is the same as the 2018 model. That also means your Mac mini can run macOS Mojave should you choose to do so.
Before you visit egpu.io,
READ THE OFFICIAL APPLE SUPPORT DOCUMENT ON EGPUS. Read it several times
very carefully paying close attention to their recommended hardware.
If a particular graphics card and/or eGPU enclosure doesn't appear on Apple's support document, that doesn't necessarily mean it won't work, however it does mean that Apple hasn't thoroughly tested those. Failure to follow Apple's recommendation may result in hours/days/weeks of you pounding your head against a brick wall because you wanted to shave $50 by trying to use a second-tier AIB's graphics card. Your call.
Basically, Apple says that you want an AMD reference Radeon RX 580, a Sapphire Pulse Radeon RX 580, or the AMD reference Radeon RX 5700 XT (the latter if you are running Catalina). The eGPU enclosure ideally should be the Razer Core or the Sonnet Breakaway.
I assure you that the Sonnet eGFX Breakaway Box 650 and the Sapphire Pulse Radeon RX 580 card combination is an effortless no-brainer on the Macs and Windows 10 PCs I have tried.
If you don't need the best 3D performance, you might consider picking up the Pulse RX 580 sooner than later. Once AMD announces their Big Navi/RDNA2 GPUs, inventory of the two-year-old RX 500 series will start to dry up. Note that Sapphire no longer markets the three-year-old RX 400 series cards. Some members of the RX 5000 family will probably be around for another year.