I agree with many here saying that you should keep trying to learn to shift... I'll even try to help. Shifting/double clutching is about rhythm. Do you like music? Find a song that is mid tempo or even a slow song with loud drums.. Now.. use your left foot to tap to the deep thud of the bass drum... and your right hand to slap your right knee on the snare drum crack... Once you get this down, instead of tapping down with your foot on the bass drum beat... Lift s pillow up with your left foot and still slap your right knee with your right hand... The point is you're building the same kind of coordination needed for double clutch shifting.
If you decide to go down the route of getting your class B to drive straight trucks OTR, check out "Expediter Services" and "Panther Expedited". They are both companies that hire newbies with class B CDLs to drive as a team with another driver in a straight truck since your wife is no longer interested getting her cdl. I would be doing that right now, however, I don't team and just want solo. If I decided to do that then I might as well go the class A route and put up with a trainer for 6-8 weeks "teaming" with the goal of being solo as the reward....
At least the good news is Freightliner M2's have syncros in their gearboxes. So you can shift them like a regular car.
Some do... the M2 is available in configurations from a Class 5 single axle up to a Class 8 tandem... you start getting into the Class 7 trucks (especially single axle trucks set up to haul semi trailers), then you have the option of getting a twin countershaft 8LL, 9 speed, 10 speed, etc. in them. A single axle expedited would probably have a synchronized 5, 6, or 7 speed transmission, but it's not guaranteed.
Nobody has started out perfect shifting a non-synchro'd transmission. I picked up upshifting easily but downshifting took me a few weeks to get it right; once I figured out how to listen to the engine it made everything that much easier. When the engine drops RPM's in whatever gear and stops making "noise" then it's ready for the lower gear. Just tap the clutch a bit (don't PUSH), toss in neutral, rev a bit (each truck is different), tap clutch again, then throw into lower gear. Pretend the clutch is a hot plate that'll burn your foot if you leave it on there too long while the truck is moving. The less clutch the better! And no, even Class A drivers are restricted to automatics if you don't test in a manual. It'll be on your license along with whatever other restrictions you may have.
My wife did her road test with an automatic transmission and she does not have a restriction on her CDL-B.