What phone is carrier, bootloader unlocked with removable battery

GSMArena's "phone finder" tool may help you here. I did a quick search with it, just specifying "Android, removable battery, released since 2020, 4G (any band)" and got 33 results. If you follow that link you can "refine" to your taste, including specifying the LTE bands you want for compatibility with Sprint (click on "4G" in the search options and there is a set of tick boxes). Of course it would still be worth checking that there are no other carrier-specific quirks or limitations beyond just supporting the right frequency bands, such as policies where they will block handsets that aren't on their list even if they could technically work (something that I've heard some US carriers do).

What that search won't tell you is the bootloader/rooting stuff. No mainstream manufacturer sells their phones with the bootloader already unlocked, so the question is which are easy to unlock. Best bet might be to draw up a shortlist then either ask again or check the device's development forum over at xda-developers to see whether there is any recipe for rooting. I seem to recall that Nokia aren't very root-friendly (relevant because budget Nokias made up a lot of my search results), and I have the impression that in the US even non-carrier Samsungs can he very locked down (European models are easily unlockable, but I would check very carefully for Sprint compatibility with those!).

"Removable battery" is the problem here: that really limits your choice of phones these days.

 

I feel your pain.
I love removeable batteries, and unlocked phones.

I have never rooted one, so I can't say anything about that.

As far as Sprint goes, they were my main carrier until T-Mobile bought them and shut down Sprint's working system and replaced it with T-Mobile's mostly disfunctional (trying to be nice here) system.

You are lucky that it lasted as long as it did.

Not just the device, but the service.

Anyway, it seems to me that nowadays if you want a removeable battery, it pretty much regulates you to the lower end devices.
Not that that is a real problem for me, as that is what I can afford.

The anticipated lifespan of ANY device is 2-4 years, so even if I had the cash, why would I bother spending more than I need to, right?

At any rate, a manufacturer unlocked device is the most compatible with multiple carriers.

A good one at a nominal ($100 - $300 USD, depending upon device) price can be had at Wal-Mart.
Don't get me wrong- I am not a fan of Wal-Mart- but things being what they are, it is what it is.

As far as a replaceable battery, I have gone that route a few times, and it is great- right up until the carrier changes technology and the device that has a new battery can no longer make calls.
Oh well, they still work on Wi-Fi.

Rooting is not what it used to be.
Most new devices (all that I have seen, anyway) give plenty of leeway in disabling bloatware and unwanted apps.

There are apps such as Greenify that allow you to automatically force stop apps when you are not using them.

https://greenify.en.uptodown.com/android

There are programs for your computer that allow you to make apps 'disappear'.

https://appcontrol.neocities.org/index_en.html

With these controls and the built in options of modern Androids, it becomes rather easy to make a battery charge last throughout the day.

You mentioned that you want a 'carrier' unlocked device.
That is something that I do not recommend.
A carrier unlocked device will most likely only work on the system that the carrier uses.
It was, after all, manufactured for that carrier.
Multiple carriers all use the same systems, but here is the problem- when your Sprint goes bye-bye, and you are stuck with T-Mobile, you may not have a working device because of weak signal strength.
(Ask me all about it.)

Now, a manufacturer unlocked device will be different, as it will be capable of working with all or most of every cellular system available in your country.
Yes, they cost a bit more, but they are way more capable.

Here is how it generally goes.
You have let's say Verizon, and you get the device unlocked by them.
Great.
But now when you want to leave Verizon, you must use a carrier that uses Verizon towers.
That is usually alright.

Your situation is a bit different, because the network that you use and love is going to be shut down, and gone forever- no longer in existance.

The new network may or may not (as in my case) work in your area, or where you normally had good service.

If you have a device that is carrier unlocked, then you very well may be stuck using the same crappy service even if you change carriers!

On the other hand, if you have a manufacturer unlocked device, then you are not restricted to only one system.
Even most of the remote areas of population are serviced by at least one cellular company reliably.
A manufacturer unlocked device gives you the option of using an entirely different cellular system from any carrier.

From my own experience, this is what I recommend to anyone.

Your old device will continue to work fine on Wi-Fi, by the way.

 

Also look here, https://forum.xda-developers.com/
You'll want a device that devs have worked on, have made custom roms so do your research first