Storage in an iPhone is strictly non upgradeable. I would probably be looking at more storage as a baseline. 128gb as a minimum.

The 12 is a solid device and will probably last you a long time, especially if you’re willing to replace the battery.

If you can afford to pay more, the 13 has better cameras and battery. The 14 is not a particularly good value proposition over the 13 so only consider it if you can get a good deal.