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.

Depends on if you can wait 6 months on your current phone. There’s always going to be another phone on the horizon, and the 15 is all rumours at this point.

Upgrade now if your current android is going to be a severe detriment to your life, the 14 is no slouch. Wait if you can wait.

Money

Depends on how long “a while ago” is and where you bought it from. If you’re outside of a returns period your best bet is probably to sell the 14 and buy the 14 pro with the profits.

You could do a trade in through Apple, but I don’t think you’d recover as much of the value as selling the phone.

iPhone 11, it will be supported the longest. The only concession is the fact that the 11 has a LCD screen vs the OLED screen in the XS.

No they hold value pretty well because they’re supported for so long