Myself a noob rider, i'd prefer two cyl than one when on a highway. I always feel tired when on a cub doing 100km daily. But on 2 cyl abit less.

I travel frequently on my Dominar. Although it can do 130 rather comfortably and top out at 160, the big single is a vibe fest and adds to fatigue very quickly.  Twins will do much better . There are cheap big cc alternatives.

You don't need an ADV to get that upright riding position. ER6F or the naked ER6 or Vulcan 650. Both can be found on the used market from 10k to 20k. All you need are handlebar risers on the ER6 to get your upright riding position. Just add a windshield on both for comfort against wind. The other option would be the dominar like I have, easily found used low mileage examples frequently around the 8k mark or less. I did 40k in just 20 months (including 3 MTHS of MCO)...so that's a fair bit of longer distance highway rides. There are options for the smaller 250 twins from Kawasaki. Just get the naked z250, add a windshield and handlebar risers to it

You really need  a lot of patience to do longer distances on a smaller bike. Extended travel times will be similarly trying and tiring. Take frequent breaks. Low speed long distance travel can be boring and may affect your concentration.

Honestly, I don't consider anything smaller than 250cc to be "highway capable"...it's not what top speed the bike can do, but how long can you keep running it at 100kmh+ max rpm for extended periods of time. On a small bike, you are pushing the bike nearly 100% for the entirety of that 5hr ride to JB.

Your 450km mix balik kampung is similar to my recent trip to Betong. With only 3 stops ,(for fuel) .it isn't what I call a comfortable ride.

If you already have a pulsar and fine with it, why not use taller handlebars or risers, and angle your handle bar closer for that more upright riding position? Assuming you have enough cable slack. Adding a windshield makes a huge difference in highway comfort.