I have to say that I don't feel as up to date as some others on this site so take what I offer with a grain of salt. I had my surgery 25 years ago and was symptom free until a few years ago.
I couldn't muster a bowel movement, pushed so hard that I ended up in the hospital on vacation for a couple of days. I started in pelvic floor p/t after that, which lasted quite a while. I've been in and out of p/t ever since, although I've gone 9 months without it, which has been great. I credit the Metamucil and self dilation for that.
From a recommendation on this site, I use Metamucil orange flavour, full sugar formula and only add a bit of water to the 2 tablespoon dose (not the 8 oz recommended on the canister). I recommend that you aren't sedentary immediately afterward (i.e., don't lie down to watch tv or kick back in the recliner). I tend to drink some additional water fairly soon after drinking it to help it work through my system. (Early on, I drank some, reclined on the coach to watch tv and it felt like it was expanding in my chest. It was pretty scary. I stood up, drank some water and was totally fine a few minutes later. I don't want a repeat of that so I drink it, clean up my kitchen or whatever for a few minutes and then I can go about my day.
My stools have a bit of substance now, which is remarkable.
An anal sphincter is meant to have flexibility and be able to handle a full size bowel movement. Mine has only dealt with fluid for 30 years, which is why my GI hypothesised that it had "shrunk" (my term, not hers!).
Hope that makes sense. Trying the Metamucil can't hurt you ... and may make you significantly more comfortable.
Good luck!