Not an expert but I've been buying cheap ingredients whenever in town. And focus on the types of potions you want to make like healing and experiment to find which ingredients heal.
I am going full out on alchemy and taking the perk that allows you to identify 2 then 3 then all 4 by tasting. Then I will get it to 100 and make it legendary and level it back up. But AFTER I identify what pretty much every ingredient does in game. I can't be bothered looking things up online. And this would be irrelevant if you are not playing to legendary levels. I am. I am going to try to do most of the content with one character.
Third thing that is a tip is that you can buy training at the vendor in some places. Then that vendor has a lot of money and you can sell them all of your expensive yet useless potions.
Fourth tip is that there is a perk that allows you to harvest double when you harvest flowers. Doesn't seem to work on all mushrooms and only in harvesting not vendor sales.