In my experience, older people want to use their smartphones forever so that they don't have to relearn how to use them. If I were you, I would therefore use a mobile phone with the longest possible software support. I think that the Google Pixel 8 fulfils your criteria and offers very long software support among Android smartphones. Given Google's size, you can also assume that the company won't go bankrupt before then.
In my experience, older people want to use their smartphones forever so that they don't have to relearn how to use them. If I were you, I would therefore use a mobile phone with the longest possible software support. I think that the Google Pixel 8 fulfils your criteria and offers very long software support among Android smartphones. Given Google's size, you can also assume that the company won't go bankrupt before then.