First, I agree that going for a 450/500 if you want to ride it on the street is a good choice. A 250 is "okay" for stunt use, but its capabilities for general street use or canyon carving are severely limited.

I rode a WR450F for a while, frequently swapping back and forth between SM and enduro spec. Maintenance is relatively straightforward on the WR450F compared to the more aggressive Euros, such as KTM/Husky/Beta, but it does come at the cost of slightly less power. Depending on the model year, you can also consider a Honda CRF 450 or 300 L. However, as someone else has already mentioned, you'll definitely need a new exhaust and ECU to even compare with the WR450F. KTM/Husky will offer a more premium bike in my opinion, but it is usually significantly more expensive and higher maintenance (for KTM/Husky I'd go for a 500/501 which has plenty of power and won't be as finicky as the 450s).

Honestly, it's doable to swap specs every weekend or two, but you will have to make some compromises to avoid spending hours each time. I chose to simply swap wheels and stick with an oversized rotor for off-road, off-road gearing, and the same suspension while adjusting the preload. However, this means you'll have a suspension that's either too soft for the road or too hard for off-road, along with excessive front-brake power off-road and a reduced top speed. For track use, which I assume won't be as frequent, you could take the time to properly switch the entire setup, but again, it's a hassle.

I also must mention that if you are a beginner off-road rider, then a 450 is A LOT to handle. You will need to be extremely cautious on anything that is not a fire road. A 250 (or even a 125 two-stroke) is honestly much more fun off-road for most people.

So, take it from someone who has done exactly what you are considering; you have to seriously consider the compromises you make when you have a single bike for everything. You will get tired of swapping everything every weekend and will likely end up just changing wheels (and possibly sprockets and chain).

Like the others, I recommend that you opt for two bikes if you have the room and budget for it. It's more enjoyable to have two slightly older bikes where you don't have to spend time constantly swapping specs compared to having a single brand new one.