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.
If you use a usb 3.2 gen 2 port, your max speed will be 1250 MB/s, pretty much every nvme ssd will be able to max that. Fast USB sticks will undoubtedly be very close, but might not have the same sustained write speeds compared to an SSD (With Dram cache).
If you're just going to use it for recovery ISOs or installations it's probably not going to matter much, I regularly use USB 2.0 usb sticks for that purpose just fine.
It's up to you though, I had an nvme ssd laying around and bought an enclosure for it, I get about 1000 MB/s read and write with it.
So if you have an ssd laying around I think that would be a good option, but a usb stick will be fine as well and would be a little bit more compact, if that matters to you