Shabbat: Volunteering as Firefighter

I work full-time as a fireman and thank G-d I work a career field that has the impact on my community and people in general that it does.

Given the nature of the job, I must work on Shabbat routinely when it is scheduled, but I wonder if it’s permissible to work paid overtime if it is voluntary?

There will be times in the future where I work on an ambulance as well. More specifically given the circumstances, it is almost always voluntary overtime that could be given to someone else unless forced. Forced overtime almost never happens.

Of course I don’t mind making extra money where I could potentially save a life, but I want to honor G-d first and honor the sabbath.

Should I pass on the overtime if it comes on Shabbat? If im forced should I use legal grounds to avoid working on the sabbath due to religious reasons since it’s not my assigned day and I would gladly work another day so long as it’s not the sabbath? Or, is it all permissible given the nature of my career? Also, how can I honor the sabbath on duty?

Any and all opinions and guidance are welcome!!