I am planning to get an external storage for Ventoy use. My current random usb 3.1 stick writes .iso-files with only 10-15 mbps only. I found this webpage for usb stick speed comparison: https://ssd-tester.de/usb_stick_test.php
The fastest usb sticks writes 800-1000 mbps in the crystaldisk-test.
If you have personal experience on this, please recommend which approach is better.
USB flash drives usually use the worst flash memory available. There are good ones but they are hard to find and expensive. Going for a NVMe SSD in an USB-C enclosure gets you way more reliably better speeds and durability.