OSMAnd doesn’t always have routing limited to a few hundred kilometers. If one is cycle-touring, then the Brouter engine plugin for OSMAnd can be installed and that generates pretty quickly very long-distance routes.

But with regard to planning long car journeys, there is always the option of generating the route using one of the several OSM-based routing engines on the web, then downloading a GPX file, opening it in OSMAnd, and telling OSMAnd that is the route you want to follow.

I just add intermediary way-points for long routes. Works well enough.

Not sure why you're downvoted, it's a legit problem, especially if practically any other engine doesn't have a problem with it. In reality I think I'd just split my route to get around the problem and call it a day, but it was convenient to not think about this, when planning my 800 km drive.

I just tested, OSMAnd generates a 900 km route on my phone in one go, in about 10 seconds.

I tested it too, and ended up generating an 1000 km route without a problem. It was giving me "Ending point too far from nearest road" - until I figured out that the problem was the missing maps. As long as you have at least the road map downloaded for the whole route, it does the job.