Thanks for your kind words and sorry I’m not able to help you more. Which country are you from?
Just so you know, opening the S7 is not as easy as you might think. Glueing it shut again is also not easy and something I couldn’t manage, even with a precut sheet soecificially for that (can be bought at repair stores).
On my S7 with the hardware defect I mentioned I performed a battery replacement. For opening the phone you need a heatgun, a feasible suction cup and ideally something like an iFixit Kit. If you don’t own these tools already it might be worth it to just buy a refurbished phone for a small premium.
I can’t recommend you a handset that matches your criteria right from top of my head, but this page lists all compatible devices and lets you filter them based on different criteria.
I think the Gigaset GS290 is an interesting device, if the size fits you still. Bonus is, this device can be bought pre-flashed at the /e/ store.
Flashing and selling devices sounds like a good idea at first and I played with this thought as well, but reality is that you invest a lot of time for small profits.
You’d search for devices online, try to guess the condition they’re in, hope they’re not FRP locked, bid for them (which includes the possibility they’re getting to expensive and all the invested time was for nothing), pay shipping, invest the time to flash and the time to sell it online and ship it.
Most times the devices that are cheap are damaged and you can’t be sure if they’re reparable. It can be an aging battery or a defect circuit board. A device with a smashed screen can have more damage or can be locked, which makes it unflashable. It could come from another country, with a SoC that doesn’t have an unlockable bootloader (like the Samsung US models).
You see, a lot of risk for nearly no profit.
If someone wants to send their phone for flashing service, this’d be another case - but I don’t think this should be done for money (that’s just me, everyone should decide for himself).
Hope you don’t mind the long answer and feel free to ask if you need further assistance.