Aerated concrete, leveling the first row of stones

So I have now laid out the entire first row of stones and sanded them all to the same height with the belt sander.

I even had to remove 4 stones again because one corner did not have a 90° angle.
I only noticed that the next day when the mortar was already set.
But that is the advantage of the [Schweißbahn], wooden strip next to the stone and hit the strip with a steel hammer.
The stones remain intact and the mortar separates from the [Schweißbahn].
Also chipped the mortar off the stones with a chisel and hammer.

Now I am considering how to mill the stones from above for the reinforcement.
Doing it by hand takes forever and is tedious.

Now I am also thinking whether to reinforce only the first row of stones and place a 5x5 mesh on the following ones.

The groundwater level is now at its highest, actually I brought a pump since the neighbor and I share an old well that is exactly on the property boundary.

The well is so full that I can scoop the water out with a bucket, 30cm below the old ground level.
I raised my property by 80cm, where my first row of stones is, the neighbor has his windows.

No rainwater drains for him anymore either, and as he told me, his house also has no horizontal barrier.
The walls absorb the water up to 1.50m high for him.

Nothing but mold can stick to the walls there.

Evening, I am now at the 4th row and wonder what would be better for the concrete lintel above the windows and doors.

Mix myself or buy ready-made?

How does it look with the load when I mix it myself, how long do I have to wait before I can put the stones on it?

Of course, I would prefer to pour it myself, more iron and it would also connect with the stones.

Also, with the ready-made beams, my stone width is not available, which means I would have to lay 2 side by side.