You might try Adobe Lightroom Mobile’s camera app. It uses 3 exposures for HDR I believe, while SmartHDR uses more. And LRM is free. I think Procamera can be set to use from 2-5 exposures, but they caution that using 5 can be slow and problematic with moving subjects (see below about the buffer)
The problem with replicating the actual thing that SmartHDR is doing in a 3rd party app on older phones seems to be twofold. To use many images will require that photos are continuously made and saved to a buffer, so when you press the shutter, it doesn’t take an extended time to make the multiple exposures. Secondly, to combine a large number of images takes a lot of processing power that older hardware probably just doesn’t have. That power comes at least in part from the new neural processor.
Hope this helps.
Cheers, Azy