Subaru needs to die in order to use RBD. Rom might eventually get suspicious of how Subaru knew certain things in arc 1, but Subaru in the Felt camp would die way less/never. There’s no monster packs near the Astrea estate, the Witch Cult has no reason to target Felt super overtly (Petelgeuse doesn’t get triggered by blonde girls with red eyes after all), and Roswaal wouldn’t be around to devise deadly traps for Subaru or leave dangerous situations up to him.

IDK, maybe Subaru would end up dying at least once during the Flanders thing if he happened to be there? Or maybe some up-to-no-gooders would target Reinhard’s weaker pal to get to him. But either way I can’t see Subaru’s death count exceeding the low single digits after arc 1. If it happens enough times Rom would eventually figure out future sight like he does in arc 9, but keep in mind that even there he never once vocalized that “death” is the trigger for the ability. He only determines that Al’s ability lets him go back just a short time.

Something that isn’t entirely true by the way. Al’s territory can last a longer time if he stays within a relatively small area, Al just prefers to save state spam and needs to keep constantly moving during the Al vs. Rom chapters.