SEA is my home airport. I don’t fly DL and have had only a few flights this year so YMMV but haven’t really experienced any tarmac delays.

in terms of connection times, 1 hr seems reasonable for a DL-DL connection as the flights will likely be using the same terminal. You only need to take the train if you are arriving/departing from N or S gates and connecting to the main terminal gates A-D. I’m not 100% on this but I think DL mostly operates from A/B gates which are walking distance - no train required.

You can try using FlightAware or similar app to check the arrival/departure gate history for your flights & make an educated guess re likelihood of delays & how much time you’ll need to connect.