I think keeping the battery between 20-80% would be ideal (minimize stress on battery) and charging it using lower watts adapter (using compliant cable/adapters) to minimize heat. Set the new iPhone to limit to 80% charge. I have shortcut automations to notify me whenever it is below 20% or at 80% so I would charge or disconnect. If using fast charging, avoid using intensive apps that puts a load on GPU/GPU and increase heat or just leave it alone to charge (iCloud sync would activate when on charge). It can also depend on the quality of battery since it could be sourced from different suppliers. Sometimes, audio/video calls on cellular increase heat than on WiFi. Just keep it cool.