Yeah, the rewards from the standard missions don’t get better as you level up and do higher level missions. That sucks.
That said, there are higher level missions that do offer unique rewards. You need to be able to beat a few pretty tough level 12-14 story missions in order to unlock the missions that have rewards like soul ash. Those story missions do NOT scale up in difficulty with your unit level, so doing the 10 hour 1000 exp missions as much as possible helps you to out-level those story missions and complete them sooner.