A 36 slot backpack sounds kinda nice, but it's definitely not a priority right now; the first upgrade to 24 slots is plenty
10,000g won't buy all the vault bundles either, and there's little point buying them one at a time. Best bet is to wait until you have enough that you can afford to spend the whole 42,500g in one go, to get all four bundles at once
There's nothing wrong with saving it for buying spring crops, but if you haven't already done tool upgrades, get your pickaxe and axe to at least copper, and both to steel once you have found a bit more cash Mining will become so much faster with the upgraded pickaxe, making your descent in the mines much easier, and a steel axe will be able to break large logs. There will still be opportunities in winter to earn money from fishing and mining, plus selling the forageables you find.