this mod will keep your longarms when getting off of horses. So your weapon loadout will stay, but sometimes it will still be cleared by some cutscences.
https://www.nexusmods.com/reddeadredemption2/mods/3404
Havent found such a mod yet. The game taking away your weapon when you get off your horse in towns or camps is annoying.It messes up my loadout settings and gets me into a situation where I have to fight many enemies but with only two revolvers.