Find another installer or company that your comfortable with and ask them to make it stand alone system, as you need an installer for the app side of things
Or buy a Yale and do it yourself simple, odd you keep using the word total stranger , after all you were happy to trust him on what security you need
One of the fist mass produced detectors to meet bs 4737 1971 that was not made for one of the big companies or that you did not have to buy direct in bulk and you could buy from this new thing called a wholesaler they cost so much each one had to be signed for and the empty box returned
Most things in the late 70s had to be made in house even panels and out side bells, we even made door contacts reed switch glued on to an aluminium plate made in small batches with final assembly by the stores man
along with-
optex ex35,
texecom medusa,
gaurdall apollo
visonic fox
visonic coral
esp quest
cou;d tell you what the aritech one is but I do sort of tinker with alarms