I know this is a really old thread, but this now exists
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At the price you’re asking, there aren’t really any devices that are meant to be user serviceable. Every couple years or so, I’ll check to see if FairPhone will make the leap into the States, but so far, no luck. If you really want something that’s guaranteed to keep getting security updates and can be serviced, your best bet is an iPhone SE of some sort.
If you are having a hard time finding a phone you are not looking hard enough. There are are so many phones at all price ranges and feature sets.
The only exception is if you are looking for a flip phone.
I miss phones I charged once, twice a week even after heavy use.
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I’m considering S20 Plus it’s only a few bucks more on ebay over the FE version or the S20 Ultra which is $100 more but I’m not sure if the camera justifies the $100 price increase.
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According to the link I provided, the only S20 that has expandable storage is the S20 FE. 
That’s an interesting phone. I can see they cut corners for the FLIR camera which is a really cool feature, but I’m sure I would not need it for daily use.
This site gives clearer information and they says they all have expandable memory
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I do wish there was more power management control on smart phones today. For instance, if you’ve got mobile data and WiFi data enabled and WiFi is active, then ignore mobile. Don’t even check it.
I’d also like the ability to create an app group for settings whereby with 1 click I can enable or disable their ability to pull data. Too many run in the background checking on things that I don’t need at the moment, which has a drain on the battery.
In the beginning I was seriously upset with phone companies making captive batteries (not user replaceable). I really think casings can be designed to provide excellent integrity while still enabling user access to the battery. But… it can present a challenge to water resistance. I had the misfortunate of making a mistake that nearly cost me my phone. I got distracted and didn’t realize the phone was in my swim trunks. I jumped into a pool… and immediately I realized my mistake. I leapt out of that pool so fast. I quickly powered off the phone and then went inside to hit it with a hair dryer. After a few minutes… I tried my luck. I powered it up. And it woke up just fine. I did get a warning, though. Moisture detected in the USB port. So the Android OS kept the USB port circuit off. After about an hour? It was reactivated. And all was fine. So yeah… I think those sealed cases provide some good protection.
The other thing is, my Google Pixel 5 is now over 3 years old and the battery is in excellent shape. I can still go about 3 days between charges (under normal light use). And the smart charging works extremely well. When it needs to be fast? It’s fast! Overnight? It attenuates the charge so that it gradually charges up to full by around the time I get up. This tech seems to work really well at maximizing battery health. And in that regard, I don’t miss having user replaceable batteries now.
EDIT:
Here’s another site with some info on the S-series, though this is a United Kingdom website…
Some phones don’t officially have user replaceable batteries, but you can purchase a kit to replace the battery yourself.
For example, many older Samsung phones are like that.
I hope that in a year or two, my 2024 Samsung phone will have a kit available so that I can replace the battery (if needed.) 
There’s actually a cheaper one that doesn’t have the thermal camera and has a IR night vision one in it’s place. These are some other smaller phones that I could remember
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I’ve a Samsung A55 with MicroSD card slot. It’s okay. Probably slightly larger than you’re looking for.
I came from a cheap Motorola G31 (?) that was dying. This phone was about twice I paid for my previous 2 phones (both budget Motorola G series).
I must say I’m slightly disappointed, the phone is fast and the screen pretty nice, camera is okay… nothing is outstanding.
I find the Samsung OS infuriating, volume buttons on the side don’t do what I want (why adjust media volume when nothing is playing, it should be notification volume if no media is playing)… If I’d paid current prices, I’d be less disappointed, but the lack of competition by Motorola at the time I needed a new phone, forced me to choose another brand.
I found the Moto OS superior to Samsung as it was a much “cleaner” install, closer to “pure” Google android.
Next phone will be a Motorola (if they get back to making quality budget phones) or Google Pixel.
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I’m kinda the opposite.
I like the Samsung OS more than the Motorola OS, but then again, maybe I just like my Samsung phones more than my el cheapo Motorola phones?
My next phone will most likely be a Samsung, or perhaps a Google Pixel, but I am happy with my current smartphone, so I’m not shopping for a replacement quite yet. 
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Lol I spend half my life trying to find volume settings buried deep inside menus. I just want volume up/down to change media volume if something is playing and notifications volume if nothing is playing!
how do you deal with all the **** Samsung loads onto the phone? I wouldn’t mind so much if I could uninstall all of it! Why would I install anything from Samsung Store when I can use Google?
I’ve never had so many garbage notifications as I do with the Samsung, the privacy policy seems to update almost weekly.
Edit to add, this is One UI 6.1. for once AI does a good job of summarising my headache, this was a search 'Samsung Notification volume settings":
Wtf guys!? (Samsung). Have I just been spoiled by nice simple UI from Motorola? How do other phones adjust notifications? Like, if you go into a meeting, you look completely unprofessional fiddling with your phone trying to make the notifications silent…
1 hour later edit: okay! Hallelujah! I think I’ve found the solution (no thanks to Googles Garbage AI summary).
It appears that the default setting is to only use volume buttons to adjust media, however, this poorly labelled toggle allows the phone to behave normally- no media playing, the volume buttons adjust notifications, with media playing, it adjusts volume.
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I uninstalled many applications that came with my Samsung phone, but you’re right, you can’t uninstall all of them.
I also turned off most garbage notifications, but of course, you can’t turn off all of them. 
I’ve never used the Samsung Store because I’m used to Google Play. 
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