I have the same machine. I tried using the USB ports as an external boot drive and yeah, it wasn't much faster. What you need to do is get a Thunderbolt 3 enclosure and a fast NVMe and it will rock.
Here is what I use: https://www.amazon.com/gp/product/B0BB74BQVN/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
And: https://www.amazon.com/gp/product/B0B25LZGGW/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
Using BlackMagic speed test, I get about 2200 MB/s write and 2100 read. There are faster NVMe sticks, this is just the one I had already on hand. You might squeeze out a bit faster.
Good luck.
I think you can find better performance than what the Envoy Express is offering. As mentioned, it performs at about 1400MB/s:
https://www.amazon.com/OWC-Envoy-Express-Thunderbolt-Enclosure/dp/B08LYQPRPJ
Whereas the Acasis enclosure can achieve speeds of about 2500MB/s:
https://www.amazon.com/ACASIS-Enclosure-Compatible-Aluminum-External/dp/B0BB74BQVN
...when it's used with a Samsung 980 Pro or WD SN850. There's a picture on that page which shows a table of the speeds, and there are some user reviews with posted pictures showing similar speeds.
HTH.