I do this to spread the cost. I have a big plastic box with a lid that I put everything in otherwise I lose bags in the back of my cupboard. This year I hope to be able to keep a list on my phone of what I have bought and for whom so that when i am out and about I know what I still have to buy. The sales are a great time to pick up things.
My DS3 is an early December born and I tried to get stuff sorted early but, as I ended up having to stay in hospital quite a while, Christmas was still manic. Do as much as you are able to, it's much less stressful.
It is def not insane as long as you have the room to store the items. I wish i was as organised as you! I would probably forget what I bought. But think how relaxing it will be for you when the birthdays/christmas comes around. Stuff your dh (no offence) will he be the one doing the shopping later in the year if it isn't done now?