I got the 12 Pro and 12 Mini (for work) about three years ago. The 12 Mini is holding up fine, though the battery would be lacking if it were my personal phone. My partner still uses an iPhone XR, which is nearly 5 years old, and I’ve never heard her complain about it being too slow or unable to run an app that she wanted to use. And the XR was basically the “non-Pro” version of its generation.

Resale value wise, the 12 Pro of the same storage capacity is selling for roughly $100 more than the comparable 12 on eBay ($500 vs $400), but a 64 GB iPhone 12 is another $70 or so cheaper. (I was going to warn you away from getting a 64 GB phone, but it looks like the entry level storage is now 128 GB, which sounds like it would be enough for you.)

The max Apple trade-in values’ difference is even lower - $30 apart ($300 and $330).

So if you’re reselling, you’ll likely be able to recoup $100 of the $200 price difference in three years.

If the Pro-only upgrades (for the 14 vs 14 Pro this was an improved camera + zoom lens, higher refresh rate screen, ~10% faster CPU/GPU, and higher peak brightness in HDR) don’t have that much value to you, then it should be an easy decision to pass on the 15 Pro and pick up the 15 instead.