A couple of people asked me about my topstitching. My topstitching is far from perfect. I sometimes wish for those topstitching feet that make all topstitching come out even, but they continually fail to materialize in my sewing room. So I just sort of wing it, and it is good enough.
As for topstitching over multiple layers without breaking a needle, a size 18 needle is very helpful, but even more helpful is a very powerful sewing machine. Any machine that’s made entirely of metal and produced before 1950 or so will do. They can generally be acquired cheaply. I use a White Rotary that I bought at an American thrift shop for $20. It did not work when I purchased it, but these machines are super simple. I found the manual online and fixed it pretty quickly. It only does a straight stitch, but it will stitch through anything. Old Singer machines are fantastic, as well, but IMO the fashionable featherweights are overpriced. They only do straight stitches, but really that’s all you need. You should be doing your buttonholes by hand anyway.
The inseams are not flat felled, they are only topstitched in place. The side seams are topstitched for a couple of inches at the top, but otherwise completely unfinished. I tend to do cotton trousers this way. Perhaps if I had a serger, I would serge the seam allowances.