Do not open them! They are packed under nitrogen in the factory and far better sealed and protected from oxygen than you can manage at home. Glass, marbles, butane and all that bullshit is useful only once you have opened containers.

Refrigeration certainly helps on the mixed up solutions, but they do have oxygen available to them in the solution water. It may or may not help with concentrates depending on how well they're packed and if they're using water or TEA as the solvent, i.e. on the availability of things that the active ingredients can react with. If you're going to refrigerate, only bother with the developer parts (and maybe E-6 reversal+pre-bleach) because the bleach, fix, etc won't really go off. Handily, those parts are physically much smaller.

Powders will last the longest of course. Up to you whether the processing quality of powder kits is sufficient, as they tend to be blix kits.

The other thing is that the chemistry is all available as component chemicals. I've never DIY'd it myself but there are people here who have and I wouldn't expect it to be particularly difficult as long as you can physically buy the right chemicals.