The new iPhone SE at $399.
Or, Apple sells the XR at $499, which I think for some folks could be a better deal. It gets you a more modern form factor, should have a better camera, and will definitely have better battery life than the SE under heavy load. Also, in my opinion, the gesture-based system found on iPhone X-style iPhones is greatly superior to home button equipped iPhones.
The XR (or the 11) is on the larger side, though. The best thing that Apple sells that’s as small as possible is the iPhone 11 Pro. You could pick up an Xs or X instead to save money - but the 11 Pro has much better battery life than those. Still, all options will get you to the end of the day with light usage.
Don’t forget to buy any storage upgrade you need up front. Don’t forget to check Apple’s own refurbished store, which sells devices that are essentially new with the same warranty at a discount.
I don’t even think Android is a decent consideration for what you want unless you specifically shop for something that works well with custom ROMs. Android software support is just too short. And it really sounds like you want a phone that doesn’t eat up your time with tinkering and tweaking, Apple fits that bill. And you want to take pictures - with Apple you basically just can’t go wrong with the cameras. They’re always simple and fast, with unopinionated results.
For small-ish phones, probably the only decent Android choices are the Sony Xperia Compact line or the smallest Google Pixel phones. Almost everything Android is huge. I believe this is due to their poor battery optimization where every device needs a 4,000 Mah battery just to get through the day.
The Pixel 3A could work for you but it isn’t sold with more than 64GB of storage and it’s a lot slower than the new iPhone SE.
Apple will get you at least 5 years of software updates (which is on an increasing trend) with their phones since their release date. They have a high resale value and migrating from one to another is a breeze. Even when you lose software updates, the App Store and most apps continue to work for a long time. Apple even recently patched FaceTime so that it wouldn’t break iOS 9 compatibility (which is 4 versions ago). In contrast, the original Pixel just lost software updates, while its Apple contemporaries remain supported.
While the idea of owning a long-lasting phone like a ThinkPad is appealing, I don’t really think it ends up making a lot of practical sense just given the fact that phones aren’t really sold that way. I think that, on an economic level, the best way to have a reliable supported phone at all times might be to simply buy Apple’s 2 year old phone every year, and then sell it the next year in favor whatever the next 2 year old phone is. If you do it that way you essentially avoid the frontal depreciation and you’re probably spending $100 a year or less on a phone after you sell the old one, and never losing software support or running with a busted up phone.