I looked at my tyres today and in between the tread there's more cracks than i've ever seen in a tyre... x 4

Not sure if it's safe or not driving with the tyres in this condition... i'm guessing not.
Before I die in a flaming wreck I guess I better put tyres on the priority list.
What's good for the road, sand and bush trails - in that priority? I'm not a hardcore 4x4er, so 33 inch muddies aren't gonna suit me too well. Most tyre posts seem to focus around muddies, but like i've said, they're not for me. Something good for the beach, getting to campsites, and probably most importantly - commuting.
Currently got BFG 215/70/15s (maybe 75s, can't rememeber) - they're good tyres, plenty of tread left, but no idea how old they are. Maybe I should just replace these?
I know nothing about tyres, so any advice appreciated.
Also - can I replace the smaller spare wheel with one of these BFGs? Not sure if the tyre mount will fit the size...