The question:
"How well does an external SSD-USB work in real life?"
My answer, based on personal experience:
Very well indeed.
I've been booting and running my late 2012 Mac Mini from an SSD in a USB3 docking station since the day I took it out of the box in January 2013.
It ran very well then, and it continues to do so today, going on 5 years later.
I daresay I have more experience at doing this than any other poster in this forum.
Having said that...
There is NO WAY an externally-connected USB3 SSD can compare to the INTERNAL SSD's that Apple is installing into their iMac line. Not even close. Realize that before you begin.
But... for "extra external storage" (NOT the boot drive), an SSD in a USB3 enclosure (that supports UASP) should yield read speeds of 420-430mbps and writes in the 350mbps range. That's still very good (those are the speeds I get booting/running from an external SSD).
So... re your question:
"How well does it work? It will be absolutely silent, of course. A 256 GB main SSD would be enough for all my programs and the external 512 enough for all my data."
I think that will work very well.
Use the internal SSD for the OS, apps, and your basic accounts.
Use the external USB3 SSD for storage of larger libraries (movies, music, pictures).
Once set up properly, I doubt you'll have any complaints.