My friend and I haven’t bought any hats (the only thing left in the store) for 2-3 months, and it was enough to buy everything that we didn’t have (3 frames and 5 skins from Geheimnisnacht 18-21). I think you could do the same before each event (Quiet Drink, Skulls) to buy all at once
There’s also a nice guide on Steam to track frames/skins (and check paintings in this guide, maybe you will be able to buy them in the future too)