It’s such a hard question to answer with a definitive answer, but some factors are vehicle age, condition and mileage (obviously). I say age, because age means rust/damage/danger, for some of us living in certain parts of the country. A fifteen year old vehicle in the rust belt that hasn’t been maintained/undercoated, could very easily have dangerous (and sometimes very well hidden) damage to the frame, subframe, suspension mounting points. A thorough inspection is needed.
I say condition because there are things - despite a well maintained vehicle - might have going wrong that you just won’t/don’t know about…a small spring clamp on a throttle body coolant hose may have let go (or about to), or a side tank on a radiator might be about to leak at the seal, or a wheel bearing might be set to go, or a brake caliper, whatever. You just don’t know, you can’t predict as these things get up there in mileage or age. Unless you’ve changed every single part on a vehicle, you just don’t know. You can predict, you can calculate, but when you’re 700 miles from home with the family in the car, anything can happen. It’s an old, high mileage vehicle (well maintained or not).
Lastly, I say mileage because that is wear and tear. Like it or in not, those are revolutions, movement, stress on parts designed to fail at some point. Use will do that, high mileage parts have a lot of use, maybe not abuse, but it’s still use. It’ll fail (at some point).
Having said all that ^^^ I’d feel “comfortable” driving 200 miles one way, max. Only because I have to drive another 200 back, and 400 total miles away from home, out on the road on a car with 200,000 plus is something I’d feel totally fine commuting 600 miles a week in, but not traveling 600 miles away from where I live (with family in the car). It’s not like you couldn’t get a tow, or get yourself to safety, but it’s the risk and aggravation of having to worry about it, or heaven forbid actually experience it. Picture your family sitting on the side of a highway waiting for a tow because your 225,000 mile axle broke in half doing 70 mph. Suitcases out, kids standing back on the breakdown lane. Police lights flashing, officer looking at you like…you couldn’t have used a newer/better car to transport your family in on your road trip? Then you’re looking for a hotel, mechanic, and rental car. Trip is over before it starts. No thanks.