I've written to my MP twice in my life (as you say, non-templated because otherwise it's as meaningless as sending them a photocopy) and got a detailed response twice. If you take the time, they generally do too. (Obvious caveat that this is a sample size of one person and two emails / letters...)

I've written to an MP maybe 10 times in total and I would say 80% responded with a template. But they do at least have to have an assistant read the letters and pick the right template response.

The one I remember where they didn't do that was when I wrote to them saying that Ordnance Survey's maps should be free, and that did actually improve! They're not totally free not but they are much freer.

And the worst response I've had was the most recent when I wrote about the UK's insane criminalisation of term time holidays, and they wrote back assuring me that they were doing everything they could to deal with COVID??

Even so, still much more effective than petitions.

For others reading, I just used https://www.theyworkforyou.com/ to write to my MP.