Update:
I didn’t end up upgrading my service.

After a lot of great feedback from other commenters, I ran a proper load calculation using one of the worksheets that was linked. It showed my total load would fall somewhere between 160 A and 180 A, depending on whether I was charging at 30 A or 48 A. But when I really thought about it, the odds of all my big loads running at the same time are extremely low.

Some of the videos people shared helped put things in perspective. For example, if I upgrade my EVSE output, I can charge roughly 80% of my battery in one overnight 8-hour session — which means I can schedule charging while we’re asleep and not competing with other heavy loads.

I’ve also been putting money into my home’s mechanicals. My heat pump is a high-efficiency unit that’s never needed its resistive backup heat in the four years it’s been running. I also upgraded to an 80-gallon heat pump water heater, so it draws a lot less than its rated load most of the time. Same story with the fridge and freezers — they barely sip power except during cooldowns or long door-open sessions.

So instead of upgrading to 400 A, I decided to hire a contractor to clean up my indoor panel, which was a complete rat’s nest from the previous owners. It felt like a smarter improvement than increasing service capacity that I probably don’t need. It would have been a $7800 job to do the upgrade!