you just need to BOOT first from the microSD …
i’ve recently tried this with an usb 3.0 to microSD thumb stick adapter from Kingston + a Samsung microSD 8GB and was succesful at booting into an ubuntu 20.04 LTS RC (release 0 from 23rd april 2020) LIVE environment and after that i immediately succesfully performed a clean install on bare-metal (asrock x470+AMD ryzen APU 3200g on internal WD blue SSD sata m2)
all this was possible on UEFI with the standard 3 partition scheme
1st fat32 as GPT > /boot/efi min 512 MB
2nd ext4 as GPT > / min 20GB
3rd ext4 as GPT > /home - make it however big you like 
and encrypt it
you can use ZFS or BTRFS instead of ext4 but they are not ready for prime time yet imo
you will get it to boot by entering UEFI after the computer starts and selecting the “temporary-boot-overide” option or something like that if you have some other UEFI closed-hw …
i’m just giving these instructions because that’s what i’ve had to work with but if you can use pureBOOT or coreBOOT on some open-hw that’s available from the FSF or other distributors then that would be a huge plus for you and for anybody else who can do so …
mind you at first i’ve tried with debian stable 10.3 and it didn’t work out-of-the-box because the bundled MESA graphics stack in the linux-kernel 4.19.something is for the older generation 2200G and 2400G APU ryzen … just sayin’
you can get the microSD card bootable by using the GNOME-disks > restore-disk image from the hamburger menu if you’re on that DE