Hey, welcome! I hope you enjoy your time here and I'm sure you'll find us a warm, helpful bunch.
Hmm, if you're looking for long-term stability but don't need to have market-leading specs, I'd recommend an Android phone that is
as close to stock as possible. This is because you're more likely to receive the latest Google updates (i.e. be supported by the manufacturer for a long time), ensuring your phone can be used for years on end. Other features to look out for are
replaceable batteries (since battery degradation is a very real thing - just look at
Apple!) and
build quality if you drop your phone a bit.
-
Google Pixel or Nexus - Obviously, Google's own offerings will be the closest to the stock experience. Since you don't need the latest tech and I'm a little sceptical of the durability of new models, I would suggest going for the original Pixel, which is light, minimalistic yet very fast and solidly-built. The Pixel is also fantastic if you like taking photos, if you're an avid Google user or if you like pretty cases!
You could also go for a Nexus, ideally 5, 6 or 6P. They're older but feel more solid and are pretty powerful. Figuring out who made what can be a little confusing, so I recommend checking out Wikipedia if you're interested
OnePlus One or Three - The One might feel a little outdated (e.g. 1080p screen) but it has definitely stood the test of time. Many customers report that it functions pretty well, even though OP is onto its 'fifth' generation!
Alternatively, you could get an OP3, which is the oldest model OP is currently selling. It's very light, sleek and bloat-free. Similar to Google, you have all the freedom you might want with customising (or not customising) your phone experience
Essential phone - This is definitely a bit of a gamble but considering it isn't identical to all the other flagships, it might be up your alley. In particular, it comes with stock Android - not surprising since it's developed by one of the founders of Android - and it's modularity should mean increased longevity and manufacturer support
Just don't buy it from T-Mobile 
- Other brands to consider: Sony, ZTE, Nokia (the king of durable phones), Motorola. Note that the phone doesn't need to have a replaceable battery since you can take it to a store and have a professional replace it for you every few years. Additionally, if a phone doesn't come with stock Android, you can install a bootloader (if you're willing to go into that) to ensure long-term support since the developer will likely support it for longer than the manufacturer.
Also, where did you buy your phone? Is it linked to a carrier? I'd suggest looking into
phone trade-ins to see if you can save some money. You might be limited only to Samsung phones, but it really depends on where you bought your phone.
Best Buy,
T-Mobile and
Tesco are some companies that offer trade-ins.
Once you've figured out what phone you want, I highly encourage you to consider a
secondhand phone.
Swappa has
good return policies and tends to be a little cheaper (maybe 5-10%) than retail price. It seems
a little safer than eBay, though since you're not looking for the latest model,
eBay isn't a bad place to look. Just make sure your purchase is covered by their
buyer's protection!
Perhaps the safest option, however, is to buy a
certified refurbished phone from Amazon. This works especially well when looking for older models and you can rest assured that the phone is almost like brand new - the only real difference is that it probably won't come in a branded box! If you're not buying from the phone manufacturer's Amazon account, do make sure there's a money back guarantee before making a purchase.
Alternatively, you can always
sell your current phone to save some money or pass it down to a relative if you're feeling generous.
