All side activities must be completed on the same save file. However, you can create a New Game for the missing miscellaneous achievements.
For 100% game completion you will need the following:
- Complete all 21 Main Missions
- Complete all 24 Side Missions
- Win all 20 MMA Fights and all 5 Horse Races.
- Find all 8 Legendary Horses, 20 Horseshoes, 9 Musical Notes and 16 Weapons.
- Complete all 7 levels of the "Body Collecting", "Stayin' Alive", "Cargo Transport", "Pony Express" and "Taxi Guy" side activities.
- Complete all 5 "Murder Fests" side activities.
Location of the 5 "Murder Fests" activities :
https://images.steamusercontent.com/ugc/1852677070303652316/...Video guide showing the location of most side activities :