It sounds cheesy but the most sustainable phone is the one you already own.
Another thing that might need upgrading to extend the life of your existing phone is the operating system. Phone manufacturers often stop providing updates after only a few years which can leave you vulnerable to security threats and unable to install or use certain apps.
If you have a popular Android phone you could upgrade to Lineage OS. This is an open-source Android distribution that closely resembles standard Android and it’s kept regularly up to date.
The installation instructions may be a bit daunting for many people, however if you have a Samsung there is a free tool called Odin3 which provides a simple GUI on your laptop to simplify the installation so you don’t have to use the command line. There may be similar tools for other brands.
By running Lineage OS on my phone I managed to improve the performance (by excluding unecessary bundled apps) and extend its life by over a year. The only downside was the reduction in camera quality because the original software used proprietary features that were not open source (again this will vary by model). But this is a small price to pay for 50% more life.