I have come to the realisation that preparations for post-cataclysm survival are much like preparations to travel to another planet.
Firstly - you're going to be in your little shelter for a long time, and you don't quite know if conditions will be exactly what you're expecting.
Secondly - you have to make sure you have everything you're going to need once you get there, or at least be able to make what you need out of what little there is.
If you're planning to be there for a while, you better be able to grow your own food, including while you're on the way, even to the extent of planning what essentially will be an ecosystem. It's not enough to pack lots of food - at some point, the food that you need for an indefinite stay becomes more than you possibly could take with you. So, you begin to examine first the easy things to grow, then realise that you need much, much more to make it happen properly. Of course, in a small shelter/space vehicle, your options are limited, you make seed stores, perhaps some smaller animals, and hope that they all last long enough...
Or, you go big, make lots of space to get a range of basics going all the time - fish, grains, even trees (at least trying to keep them from getting too tall), bees possibly. You begin to realise that it's a lot of work for one person, of course, and a single person may not have the capital and time to bring it all together.
Moreso if you want a secure, underground shelter - anything large and good is not going to be cheap.
I mention bees, and immediately people would think of european honeybees. There are, however, a number of other species of bee that are much smaller, and more fortunately do not sting - making them much safer to handle in a confined area. It also means that you have less to get - beekeeping is expensive if you think traditionally.
In short, you start thinking much along the same lines as someone planning to send humans to another planet.