I once thought that the best strategy for what caliber to pick was get the ones that were most common because since there were more of them out there you were more likely to find ammo. Now with the current shortage I am not certain. The local stores have been out of all the common stuff both ammo and components for months however I can buy less common things with no problem.
.45 ACP is out everywhere but no problem with finding .45 Colt.
.308 is gone but I just bought some .45-70 and had a choice of loading.
Unique powder have not seen a can, Black powder, what granulation do you want?
The Old cartridges like .45 Colt and .45-70 were originally loaded with Black Powder and Cast Boolits and could be again. Black powder is easier to make than smokeless.
The guns that shoot them were simpler to make and repair and were more forgiving of variations in what they were fed. Might be better to grab a Colt peacemaker and leaver action rifle than a wonder nine and black rifle.
You would be giving up some things in return for others. You just have to figure out what is most important in your situation.