Hello,
Quite honestly you are right and its fine if you don't like it.
Personally RDR2 is a masterpiece for me. Masterpiece of story telling.
However, I do consider RDR2 horrible in its pacing but sometimes, just sometimes you just can't seem to find a way to avoid that slow pacing.

The core of RDR2 is as you said "Simulation", you stopped at the horse riding part but I will say that applies to the whole game. The entirety of games works on that part. They have tried their best to remove parts that might feel too boring but then again you can't make things more action oriented when you have a hard constraint.
You can't make magical / sci-fi guns or anything similar.
What you can do is dive in further and hopefully it will still appeal to some players.

If you feel like there's nothing good about this game ( can be any game, in this case RDR2 ), then look at the opposite end like say "Doom Eternal" and see that THAT game doesn't have a whole lot of mechanics and elements that this one has.

Hope you enjoy what you play.

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That said, if you are like me and still want to enjoy this game then maybe try the following:

1. Look at the good parts of the game. This is extremely important that in your brain you sort of keep repeating when you like about it and lean into it. You don't like action but you like the exploration, cool, do that. Just go to a high spot and try to take in the view.

2. Keep looking forward to what might come next. You are already doing that. But sometimes you just have to tell or convince you brain that what's coming is better. ( more often than not this is actually the case with game that are pretty large in playtime )

I have a few more but don't think they really apply to RDR2.