Riding off road, whatever that means to you, the primary issue with a low muffler is hitting it on rocks and road debris. Don't let the low exhaust stop you from going to the places you want to go. If you're exploring the places that I think of as "off road", including river crossings, you may want to cobble up a belly pan to protect the oil pan, especially on a moto with a lower ground clearance to the oil pan. The rougher it is, the more important are aggressive tires and lighter motorcycle weight.
I seem to recall you were in Peru/Colombia during the flooding a few weeks ago. Did you ride the dirt mountain roads of Peru on your 150cc moto? You are the one who can answer these questions. There is no perfect moto, and every choice has advantages and disadvantages.
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