It might be. All I can say is that installing and booting externals is a very varied experience, between different people and also for the same people at different times, as software and firmware is updated.
Also, for me, a repeat attempt doing exactly the same thing is sometimes successful. This also applies to some of the messages that can appear in the process. Shutting down and rebooting the message is sometimes cleared.
In general Thunderbolt externals are much more successful than USB. (BTW the Sandisk Extreme Pro is USB not TB3 as your thread till implies).
For me, at one time it was essential to have the target external directly connected, not through a hub.
Also for me installing from a bootable USB installer has generally been more successful than installing from Recovery or from the installer file on a local location. But I have succeeded from all three ways.
I have not found it necessary to change the Startup Security to Reduced Security. This seems to be a popular belief carried over from T2 Intel Macs where reduced security is essential.
