Recommended disk format for external SSD running as system drive for a 2019 iMac I'm setting up a new 2019 iMac with an internal Fusion drive. I have an external 1TB Samsung T5 SSD drive that I'd like to use velcroed to the back of the iMac as the main system drive in order to improve performance. 

Should I format the SSD to APFS or HFS+?

The iMac has MacOS 10.15 Catalina installed by default. 

(I've read some stories of 2 minute wait times after restarts using APFS on external system drives, but I'd like to use the most up to date system if it's right to do so).
Only APFS is supported for running Catalina. There may be some weird workaround for running Catalina on HFS+, but I don't know of any.