It's unclear how and what the T2 security chip will do if you roll back to an unsigned Mojave copy; meaning that what you made with the USB installer is an unsigned Mojave copy. The T2 security chip design was to prevent booting from an unsigned operating system, either from an external drive or from a network boot. You can bypass this security feature by setting it to medium or low, but you'll lose a number of security benefits when future software demands it and with the T2 protection, some software may inadvertently be blocked even if, as you said, is capable of running in 64bit. Sort of like the iOS devices; you can move forward but you can't roll back to the previous version easily. So if you start out with a signed Mojave copy, then updating to Catalina and onwards is best.
I skipped the Mac Mini 2018 model and actually bought a used Mac Pro 5,1 as a stopgap measure. I didn't find the 2018 model inspiring as they are using the 8th gen Core i series CPUs and the iGPU on the 8th gen is uninspiring. The 10th gen Core i series CPU, however, is something else. And if the refreshed Mac Mini uses the 10th gen CPU, then I will be tempted to update to that Mini as the iGPU on the 10th gen is capable of doing AI stuff that some of my imaging applications are already taking advantage of with the PC version that supports the 10th gen Core i series CPU. Which means, I don't need to rely on my AMD RX580 to do the compute GPU work that my current AI imaging apps are using. Besides, the Mac Pro 5,1 with Mojave will receive security updates for a few more years; enough time to wait for the Mini to be refreshed with the 10th gen Core i series. Yes, the internet is a pretty unsecured place to be, but the micro-code worries with the Mac Pro 5,1 should be addressed at least with whatever security updates Apple can work with.
