Greentext

Defining your character also isn't core, since most JRPGs have a fixed character you play as.

But yeah, it's a matter of "which genre is this closest to." RDR is closest to an action-adventure game, because the core gameplay loop is on a mix of action (skill-based gunfights) and adventure (interaction with set pieces). RPGs tend to have a core gameplay loop based on character progression (leveling up, ability unlocks, etc) and interaction is generally with character abilities.

RDR's abilities generally fall under the survival/management end of things, they're interesting from an immersion aspect, but you're not actively looking to level up some ability to solve some problem or unlock some content. In BotW, you also have "levels" (hearts and stamina containers), but increasing those aren't really necessary to do anything, they just make the game a bit easier. Likewise with equipment, you don't need the master sword, but it makes things a bit easier if you have it. The OG Zelda was a bit closer to an ARPG with equipment acting as "levels" (blue and later red rings to reduce damage, white and magical sword to increase attack, etc, each of which marks a stage of progression in the game), but it's still an action-adventure because the game doesn't revolve around that character progression.

So I'd call RDR2 an action-adventure with survival/RPG mechanics, because the core loop is around action and adventure.