You want just a versatile go-anywhere bike, like a Surley or Salsa or similar - probably closest generic type is a cyclecross bike.
Smaller 26" reinforced rims (if you hit a pothole they won't cause slight wobble or bump in your ride for ever after, like 27" or 29" unreinforced rims tend to). And 26" rims give you more manouevrability.
Fat tires like 1.75", slick with tread like car tires - knobblies if you're doing a lot of off-road (you won't come off in the wet because more tire touching the road)
Puncture proof tires with reflective rim like Schwabbe marathon or continentals
Do not get suspension bike of any kind - hard to maintain suspension forks, suspension forks add weight and cost, not necessary unless you're doing hardcore or semi-hardcore off-road downhill riding.
Just a mid-range drive train is fine
Use dry wax-based lube for easy maintenance - just wipe off with a rag and re-apply
Buy new so you can get fitted correctly, your back will thank you. But you can pay double compared to second-hand. But if you buy new you get exactly what you want and it's probably quicker
New bikes can feel uncomfortable, but often it's just the saddle. I found the saddle that works for me and it comes with me on all new bikes.
I have never bought new. The second hand bikes I've had have lasted 20+years no problem. My current bike is a converted mountain bike Cannondale frame (nice welding), 26" reinforced rims (trianglular profile) from my old giant mountain bike which are still going after 20years, swapped out front suspension forks for solid forks, entry level shimano Alivio drive train which is terrible for the amount of riding I do but I make do (I really need to buy myself a new bike). I've bikepacked with this bike in foreign countries, no problem, on-road and on gravel tracks.