Best method to do your own potions:
1. Hire Brinna Brightsong as a hireling
2. Respec her to a Transmuter Wizard with max Wisdom
3. Max her Medicine proficiency.
4. Leave her in camp, saving your ingredients as you adventure.
5. Swap her into the group, giving her all of the ingredients you have been saving and let her go at it creating everything.
6. Everything she creates a potion, elixir, grenade, etc. she makes a roll on medicine, if she succeeds, she creates 2.
Because she is a halfling, she is Lucky, so has a better chance to succeed at her Medicine check.
7. After a Long Rest, she can cast Longstrider, Mage Armor, & Dark Vision, plus she can create a Transmuter Stone for someone to carry. So none of the Companions have to worry about casting those spells.
She never leaves camp, but she is a really great support character.