If you have a the Shiny Charm, Sparkling Power Lv.3, and have defeated 60+ Pokemon in a Mass Outbreak, your shiny chance will be 1/512.44. Without the Shiny Charm, the shiny chance will be 1/683.08. If you have the Shiny Charm and do the Masuda method, the chances are 1/512.44
You get Herba Mystica as rewards from five and six-star raid battles in Scarlet and Violet. However, you can increase your chances of getting even more by eating level 2 sandwiches that boost Raid Power.
By crafting a sandwich that boosts Raid Power, you can increase the raid rewards by two, giving you two more chances to earn Herba Mystica.
You should look at Source 2 for more information about farming Herba Mystica.
Source
Source 2
Source 3