As it stands today, an EV charger (or an outlet designated for EV charging) is considered to be 'in use' 100% of the time from a loading perspective (i.e., when counting how big of a main breaker you need to handle). Which, honestly, makes sense. You can't predict when other loads are being used reliably, and you can use an EV charger for many hours. For, say, during the middle of the night.. there's a high heating load so that loading will happen simultaneously.
There are more intelligent ways to handle this (and, the products exist today!), however they're quite expensive. Maybe less expensive than a full new overhaul of your house's electrical... but eventually electrical needs to be updated to code.
Also, regarding 100A vs 200A main panels: it heavily depends on the size of the house. I know in my neighborhood, which the oldest house was built in 2008, they only gave 100A panels to houses that were ~1700sqft or smaller. For larger houses, those got 200A panels.
