Well then, you would have to lock your XP around level 30 first, or lower, to complete Kalimdor and Eastern Kingdoms, since the original zones do not scale above 30. Then release your XP-lock and choose a higher level to stop at.
After you complete the EK+Kal Loremaster, it’s hard to know what level would be best for your purposes. It won’t necessarily be near-60, since in Chromie Time, everything will scale to you.
All through 1-70, the level at which you are weakest is 69 in Dragonflight. It may well be that 58-59 is a weak level in Chromie Time above 30. Much depends on what kind of experience you want. With equal gear equivalence, it could be that level 10 is the most powerful level in Chromie Time. 
You could even lock your XP at 10 all the way, buy or otherwise acquire yourself some Very Good, twink-quality level 10 gear. People talk about level 10s being OP in their first dungeons. I’m not sure how that translates to world content, but if you take a little time and gold up front to throw on the best gear and enchants you can find, you might find yourself pleasantly powerful without being godlike.
The achievements will be shared to all characters, so that’s not really a thing anymore. Reps are nice, but the achievements for reps are also shared, and the counting of reps is also shared. So while this is a nice-to-have, collecting them on one character is not what it once was. (I say this ruefully, since I collected all my reps on my Mage.
) Then the mounts and pets and toys you can buy with your reps are useable on all characters as well.
If you enjoy the DH, use the DH!
You have a tank spec Vengeance which is more than plenty to handle any same-level-scaled rares or elites in the world that you may need to face in questing. If you choose a nice level and take a little efforto to get good gear, you will rarely need it, but it is available.