I’ve played with this a lot in the past for similar hobby reasons, and you’re right that getting hold of the flight routing data is prohibitively difficult: it’

I’ve played with this a lot in the past for similar hobby reasons, and you’re right that getting hold of the flight routing data is prohibitively difficult: it’s seen as data with value, meaning it’s guarded and, if you go to official providers, expensive. Scraping is possible if you find a website that exposes the routing data you need in a format close to accessible (very rare) but it’s also complex, as the availability of each route can (and does) change at random times, multiple times per year.