[dnanian]

First, I would use a 1TB SSD for SD - about $150 US for 1TB.

Second, Disk Utility can be confusing. But, to format as APFS:

1. Open Disk Utility
2. Choose "Show All Devices" from the View menu
3. Select the destination drive hardware (above the existing volume)
4. Click Erase
5. Choose the "GUID" partition scheme (2nd pop-up), THEN "APFS" formatting (1st pop-up) and name appropriately
6. Click Erase

That works in 10.13 and later. It definitely works.