The pen tool is about the only way to go. It does take a little getting use to, but once you get the technique it goes pretty quick.
One thing that works well to see what is going is is click on the "rubber band" box. It is a drop arrow next to custom shape tool (looks like star fish).
Probably dozens of tutorials and Utube videos to help you out.
Good luck and hope this helps.
I tried using it by making the basic shape then adding anchor points to smoth out the walls... but that left sharp corners and was dificult to do
When you create a curve point don’t just click, but click and drag to draw out the bezier handles.
To change an existing corner point to a curve point alt-click and drag with the Pen Tool (in reverse just alt-click it).
Bezier Handle can also be grabbed individually by alt-click and dragging.
If you have a shape with corners (for example a Layer Mask on a Solid Color or Pattern Layer) Filter > Noise > Median could be used, but the results may not be excellent.
A little more info about the Pen Tool. Don't expect to be able to draw the arcs correctly on your first pass. The important thing is to get the anchor points in the right spots first. Then you can go back and adjust the arcs with the Direct Selection Tool (White Arrow). You may also add or subtract points as necessary.
Or, you could skip the Pen Tool altogether and use the Shape Tool (U) and the Direct Selection Tool (A).
Below is a simple green. I've used the Oval Shape Tool to create the path. It is defined by 4 anchor points.
Next, using the White Arrow, drag the four anchor points tangent to the green as below.
Finally, still using the White Arrow, hold down shift and drag the handles to place the arcs. You may have to go back and forth a little to get them just right.
Of course, more complicated shapes will require a few more points. But 4 points will define a surprising number of shapes if you put the anchor points in the right spots. And any points added after the first 4 should not have their handles constrained by holding down Shft.
Using this method, after a while, you will begin to get a feel for where the anchor points should be, and how many. Then you can confidently use the Pen Tool to place the points directly. I don't even try to draw perfectly the first time. I draw my anchor points in with the Pen Tool, and always go back with the White Arrow to finish. I must confess that I don't so much draw with the Pen Tool, as use it to construct drawings. Maybe someday I can actually draw with it.
In any case, as indicated by the above posts, becoming familiar with the Pen Tool and paths in general is more than worth the effort. Things that are hard or impossible with pixels are easy and run of the mill with vectors.