You can install macOS (this also works for Catalina) on an external hard drive (I recommend an SSD if possible). This runs at native speed and you can use it to test/try anything without touching the internal disk.
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Yes this works! Highly recommended. I’ve installed it on an Intel 660p with a USB3 1GBps enclosure.
Does the SSD drive need a separate power source when booting from it or can it run on Thunderbolt 3?
No separate power source, I've used a portable Samsung Passport 250/500 GB external SSD. Something like this https://www.amazon.com/Samsung-Portable-MU-PA250B-AM-Allurin... is similar with what I've used.