It took me about 9 months to finish the GTA 5 campaign on PS4 playing a few hours a week and I really, really enjoyed that game/immersion/world/setting/characters. Doing the same with RDR 2, I'm only on chapter 3 still but play a few hours a week and really, really love everything - the world, the characters, the music, the art, the detail, the missions, etc. Sure the combat is balls but it doesn't really matter much in SP, I got the legendary satchel thing done (took about 15 hours with zero lookups of where shit is) so now I can carry 99 of everything and dead eye every fight, who gives a fuck if I have to down snake venom like I'm Freddie Mercury and its brazilian boy jizz.
dunno why the existence of online would stop you from playing/finishing SP (unless you hated it), the nice thing vs GTA 5 is the game loads relatively fast for me on X, it does take about 45 secs to go from title screen into moving Arthur but after that there are no loading screens ever which is great.
I keep an eye out on the 7sins forum for any massive AFK MP exploits involving money/XP but so far all of them involve you putting in hours farming pelts or shit like that. RDR Online is gonna get a massive update sometime in 2019 that will roll in a ton of new shit that will make it interesting, but just like GTA 5 is a massive world with a ton of shit to do, I really don't have any interest in doing any of it.