I have the same setup on 2 pis, and even on a 3b+ I've found it's easier to just put a cheap little microSD card in to hold the bootloader than to try to get USB boot working.
The Pi3b+ can boot of USB without any SD card. Can also do this on previous model https://www.raspberrypi.org/documentation/hardware/raspberry...
but as you can read, that would make a permanent change to enable via a one time programmable bit and remove the ability to boot from SD card.
Though as you have the 3b+, then you have none of that dilemma and it will just boot, done it myself.