I considered going solo or starting a small company with a couple of friends, but I need a very steady pay check (even if smaller) and contract work is belittled in my neck of the woods (Southern Europe), so I'm currently running what passes for the tech interview gauntlet these days, with uneven results--I am either passed over solely due to perception (age, current role, etc.) or go through the entire pipeline.
Explaining that I'm not afraid of (re-)learning anything (and even with a portfolio of stuff and good references) and having a decades-old MSc seems to be looked down upon by fresh PhDs, and expectations towards specific areas of expertise seem to be unrealistically high sometimes, but I usually get through those and am eventually excused away because I'm too senior (often more senior than the interviewers or future managers, which I'm OK with but clearly raises a few eyebrows and I suspect is the main reason I'm turned away, followed by the "sales" thing).
I know there are a lot of folk like me around--how did you succeed in getting rid of the "customer facing" taint and doing a career move _back_ to Engineering?