No, there is no "offset between sensor and actual AF sensor" that AF micro adjustment is supposed to address. That is a fundamental error in understanding how things work.
AF in DSLRs can work just fine with most lenses, and then you have a lens which does not give accurately focussed results, and you then blame it on an "offset between sensor and actual AF sensor"? How does that compute?
There are three things that make for occasional AF accuracy issues with PD AF in DSLRs.
- PD AF algorithms are made to avoid endless focus hunting loops. They do not check if focus is actually reached. The are geared for focus acquisition speed, basically.
- Some lenses can do inaccurate, or "crude" AF steps when the camera is going through its AF algorithms. They basically mess up what the camera accurately says about where focus should be reached. Your Tamron 17-50mm is an example of such a lens. The Tamron service center has tried to lessen the inaccuracy of the steps the lens makes, to make it step more in line with what the camera body tells it to do. The Canon 35mm f2 and 70-200mm f4 L do not make such inaccurate steps, and hence produced better focus results without the need of "calibration".
- PD AF sensors are sensitive to certain parts of visible light spectrum, and probably even to less visible IR and UV parts of the spectrum. A lens may not focus every part of the light spectrum on the same focus plane. If just a part of the light spectrum is on a different plane when focussed, and the PD AF sensor is sensitive to that part of the spectrum, the camera can see things being in focus, when you human, who sees the whole spectrum of visible light as "one" (the image) will see the image as out of focus. This is a case where AF micro adjustment can be of interest. However, it is not a perfect solution. A lens may not have the same characteristics everywhere in its focus range. And if you follow Nikon forums, you will find that some people complain that when they perform AFMA adjustments with certain lenses on certain quite recent DSLR bodies in daylight conditions, they get out of focus results with (certain) indoor light sources.
Hope that sheds some light on how PD AF works, and why you have had trouble with that Tamron.
On contrast detect AF: it works totally differently from PD AF. With PD AF you can see in which direction you have to move to get to focus. With contrast detect AF, you just check contrast from frame to frame. If contrast increases, you have not reached focus yet. If focus decreased all of a sudden, you have passed focus and need to take one step back, basically.
With cameras with live view with PD AF implemented in the imaging sensor, like an EOS M5, they combine both ways. But the PD AF done by the imaging sensor will not be influenced negatively by certain parts of the light spectrum like you see with (some? most?) DSLRs.
Your Tamron will not behave the same on the M5 as it does on your 1000D. There are some lenses which will not focus well or at all with live view, because their AF implementation is just that horrible, but probably that Tamron is not one of those. But you just have to try that out and see. The work your Tamron has seen being done to it in service centers will not have a negative bearing on how it functions on an EOS M5, or another Canon APS-C DSLR, for that matter. Your Canon lenses will focus pretty accurately on an EOS M5 (or even a EOS M (the 1st EOS M camera).
By the way, mirrorless cameras can still deliver slightly or fully out of whack out of focus results. Sometimes that is to blame on lenses (see my remark above), sometimes on the steps a lens makes (which will account for slightly OOF results), and sometimes it is a mystery on how the camera gets to a totally OOF result (some Sony A7 models/lens combinations at times give puzzling focus results once in a blue moon, for instance).