Make sure you have a valid bios and configuration folder. You can dump your gba bios yourself and get the configuration folder from GitHub/discord. The guide site you referenced contains a tutorial for installing gbar3 on flashcards, it's actually pretty similar. It's just that you need to copy the gbarunner3.nds file to the _nds/twilightmenu/emulators folder
(correct me if I'm wrong), then create a _gba folder containing the game configuration folder, bios, configuration files (in .json format), and borders. But the most important thing is the bios, some cases when people are stuck on the gbar3 splash screen it's because they don't use the bios/their bios doesn't match. Do not use open source bios, since gbarunner3 does not support them.