No one here can make a decision that you must make on your own.
A consideration is that the X5 (plugged into the 2017) will give you performance that is "nearly the equal" of the internal SSD on the 2019. So, YES, you can save money by buying the 2017 + X5 combination, without sacrificing much speed. But it's "up to you" whether you want to do this or not.
Some folks don't mind an external boot drive connected to their Mac. Doesn't bother them at all.
Others won't even consider it.
I was of the former. I booted and ran my 2012 Mini for 6 years from an external USB3 SSD. Always worked fine for me. It STILL works fine, running as "my backup Mac" over on the "back table"...