Unless you have a lens with an imaging circle that does not fully cover the sensor (eg a lens designed for 8mm film cameras), the focal length (the "x mm" number) determines the angle of view you are getting at a given sensor size.
The "old Canon" lenses are likely adaptable full-frame lenses. 12mm (as your MFT zoom lens has) is already an extremely low focal length for full frame - rectilinear wideangles at 12mm and below for full frame are exotics, and will come at an uneconomical price even if bought second hand. Vintage full frame lenses can save you a lot of money when it comes to extreme telephoto ranges (if you can live with manual focus, size and weight), but will rarely help in the extreme wideangle range.
Your 12mm are equivalent to a 24mm full frame lens - 24mm is considered the longest "ultra wideangle" length in full frame, even a 24mm lens for a fullframe camera is almost an exot.
For MFT, in 2018, it seems that the shortest rectilinear options are also expensive - see the Laowa 7.5mm (15mm full frame equivalent) or Samyang 10mm (20mm full frame equivalent).
There are cheap so called "wide angle converters", but they either do not even try to be rectilinear or aren't good at it.
There are cheaper fisheye (non rectilinear) options at 6.5mm and 7.5mm available, but you would need to correct the distortion in software, losing some resolution and image fidelity in the process.
What could be useful for your purposes is the Sigma 10-20mm (or 8-16mm, but this one tends to be expensive too..) zoom (would need to be adapted, and will be manual focus! The "four thirds" version will not fit an MFT camera. Do not try adapting the Canon version, unless you have a very expensive/exotic adapter you will have no aperture control - adapting from Sony A, Pentax or Nikon is your best bet here...) , or the Olympus 9-18mm (native MFT, expensive).
Fisheye effects are great for art, they are not helpful in documentary architectural photography...
BTW, some technical background: P&S cameras can easily incorporate rather brutal wide angles for three reasons. First, the rear element of the lens can be placed obscenely close to the sensor, simplifying lens design. Second, the sensor can be optimized to deal with a lens closed to the sensor and covering it with a small exit pupil. Third, you can leave all sorts of lens errors in place and correct them in software and in camera - since there is no non-hackish way to even test the lens off the camera, you do not run the risk of reviewers bad mouthing your lens for being an optical catastrophe. That way, the wide angle mode on a P&S can actually be a heavily vignetting fisheye optically.
By the way, even some kit zooms for DSLMs already use electronic correction heavily (there is firmware in the lens telling the camera what to do).