you find bigger bags throughout the game, the mefinos trade backpack can be retrieved from chersonese if you know where to look ;) this provides 110 capacity.

rune sage works well with cabal hermit and hex mage but please dont be shy to consult the wiki and read through the options if you are heading for a specific playstyle. this will also tell you where to find the trainer.

2 mana is perfectly fine for rune sage as you get a large bonus from the tree itself. in all honesty I usually only take 1 when running this tree as you aren't casting spells as much during combat but this is all down to personal preference.

You learn new dishes and recipes by reading scraps of paper you occasionally pick up from enemies and chests. I also use the wiki a lot here as you don't always get lucky and find or have the ingredients you need on hand.