I've approached numerous mobile app developers and all of have no avenue for consulting or partnering to launch an application. They all charge insane fees before a build even takes place.
Where do we find you!
It turns out...that's may be the easiest part. Google has successfully gatekept that last 10 percent of launching.
In one sentence, no one (with skill) will work for you for free.
I've worked on construction software systems. Wouldn't mind an off-line chat.
One issue I see in your messaging is that construction people usually do not speak like "completely offline, no cloud, no accounts, no subscription". When doing work for SME's one of the things I enjoy most is learning systems, tool chains, nomenclature etc... The above reads like a dev harvesting for leads.