A couple of reasons to look for another solution
I thought I wasn’t flexible enough to ride drop bars, so, when I got an LHT some years ago, I set it up with mustache bars.
Some drawbacks to mustache bars that weren’t obvious before I’d put in some miles:
1. If you were riding on the hoods and not in the drops, mustache bars won’t get you much more upright than drop bars, because mustache bars don’t come back very far. To get more upright, you will probably have to switch to a bar-type that comes further back, like an albatross bar or similar.
2. This was the killer for me: Even if your drop bars were quite wide, they were probably no wider than 44 cm. Mustache bars and all similar bars are much wider. (Nitto mustache bars are 51.5 cm across.) I found that when I rode more than 15 or 20 miles, my elbows would start to hurt quite badly. That pain went away when I learned to ride on narrower drop bars.
If you haven’t already tried a shorter stem to get a more upright posture, try that. Maybe you can find a fork with an uncut steerer tube and get your handlebars higher. (Be careful, though, as LHTs switch from 26 inch wheels to 700c wheels above a certain size, so unless you are riding the Disc Trucker, you need to make sure you get the right stem.) Or you can do what I did when I just couldn’t get my LHT to work for me size-wise: I bit the bullet and got a custom frame, and have never regretted it; that frame quite literally changed my life.
I hope you find a solution that works for you.