I have had some luck with using flash drives as bootable disks by doing the following:
1. Choose a disk NOT designed for security. The reason is if you want security then the boot sector you want secure, that is NOT usable to boot anything. You want a standard storage device.
2. You need at least 20mb/second of read performance. In general faster is better but of course more expensive.
3. Once you have the flash drive partition it with something like GParted. DO NOT TOUCH THE FRONT OF THE DRIVE. Rather “shrink” the existing data partition from the back of the drive leaving at least a couple of hundred megabytes of the original partition. Many drives store critical information between the data partition and the very front “top” of the drive. There is a firmware layer on flash drives and the actual resulting hardware I think varies after the manufacturing process has ended. Tables I think are kept there. Without them things can get very slow.
4. Please note that this takes a lot of patience. Do NOT assume the process has stalled. Go away for some hours and then determine if that might be true. But the next step is to do a standard install of your OS using the area created when you partitioned the drive. Also, when updating wait a long time after updates are complete before shutting the box down and pulling the drive. They seem to like to background big data transfers and the drives are often not that fast. Give them a long time to complete what they are doing when a lot of data is involved.
5. I have not yet found a situation where they boot on USB3 – OTOH I have very little of USB3 and my machines are old. I use USB2 ports even though many of my bootable flash drives are USB3.
6. Consider using an M.2 ngff SSD in a USB3 converter box. They just seem to work but are a bit more expensive. I am using one right now.
Have a lot of fun!
Tod