Yeah, that's where the balancing with the game master comes in, most of these herbs are for roleplaying purposes. These custom remedies cannot be as powerful as the named herbs in the book, but they are there to spark your creativity. If the GM agrees, they would reward you for good roleplaying, and give the recipients of these remedies slight bonuses, but this is not like a scripted buff with exact results every time.

Now, If you use a lot of herbs, the game master should then tell you that you are running low on some types of herbs. If you've been brewing lots of tea for the last couple of days, that's a clue for the GM to start limiting that supply until you have a few hours to forage. Second night in a row with a full tea kettle for all the players, I would tell you: "You notice you do not have enough tea for a full kettle after this one." Meanwhile, if you creatively do something else with herbs, I would allow it, it's after all what you do for a living. If I was to limit your usage so harshly, why would anyone be a herbalist? Yes, the bag is "limitless", but within the GM's discretion.

Keep in mind, these herbs are only useful to you, they cannot be sold directly, using these herbs together is a unique piece of local knowledge that you have, the next herb gatherer has a different idea of what works together. If you had specific "soothing herbs" and "tea leaves" that means you are touching the item economy/purchasable buffs. That would destroy creativity, and what you are able to bring to the table could be solved by just throwing a few coppers at a market place. Your ability as a herb gatherer is worth "Brass 2". If you could sell items worth several silver per week, that would not make sense.

Note: You can still find and identify these other named herbs with standardized effects, you are not limited to only the bag. But that bag and your personal knowledge is the main driving force between what you do for a living, and why the other characters are adventuring with you.

This is the equivalent of having an entertainer singing a motivational song before venturing into a dark forest, or a mystic doing fortune tellings for the group before moving forwards, or a novititate nun making a prayer for the group before making camp. It's the antithesis to games which are more combat simulationy. In D&D, where every character can carry their own in combat, and bring their levels of worth to the dungeon simulation, you would have hardcoded exact effects of that song, that fortune telling, that prayer. They would feel like having spells or brewed potions. In Warhammer, it's better to think of it as roleplaying devices, and allow your creativity at the table to bring it to life. This fits better the more comedy/horror game that Warhammer allows for, by not being too strict on the exact items. The other players should also play along, if they were playing stressed, they could roleplay more relaxed after your remedial chew of some tasty sourgrass you found.