The first phone that would come to my mind is the CMF Phone 1, it’s a cheap (200 bucks) 5G phone where you can detach the back to reveal the inside of the phone.
The phone have been made with customization in mind, so the back is removable for you to change it as you please. Even if you’re not supposed to replace the battery, that makes it fairly easy for you to access and replace it.
Otherwise, you could consider getting a Fairphone. Even if they’re pricey for the hardware, they’re the most durable phone you can get since you can replace everything you want, whenever you want.
Here are some videos showcasing both phones if you want to check for yourself:
CMF Phone 1 (this is a review, but he talks about its main features)
Fairphone 6 (this is the last phone they released, but they also have previous phone cheaper)
Even if the Fairphones aren’t cheap, I consider that they are great value, especially for someone who want to change the hardware themselves.
I like the concept of the phone and I tend to find that years after years they’re improving. Also, I’m happy to see that the Fairphone 6 is being sold a lot more than the other one.