TBH if I do end up replacing it with something else, it doesn't have to be stock Android or even a powerful device. I'm okay with replacing my 4a with some cheap ultra-budget shanzhai phone as long as it's small and it works with T-Mobile US's LTE network.
Also the iPhone Mini itself isn't an option—not only do I not want to be part of Apple's ecosystem, but I have sensory issues and can't handle metal phones. I need a plastic phone. And no, a case isn't an option because a case would just make the phone too wide for me to hold.