Sounds like the Wahoo units will do exactly what you want, almost too well sometimes. If you go off course, it just complains once or twice then shuts up, but the maps still work and I think it will still show a breadcrumb trail so you can manually backtrack. But some of the other displays (e.g. elevation profile, list of turns, miles remaining, etc) will show nonsense until you're back on route. If you do want to rejoin your route somewhere further along, then you can use the companion app to help route you there, then reload the original route once you're back on track. If all goes well you should be able to do all this without losing your overall ride track and stats. In theory anyway.

The biggest weakness of the Wahoos, IMHO, is you can't scroll the map around to figure out how best to join back up with the route. You can zoom out a bit, but it's very limited and it stops rendering smaller roads as the coverage area increases. You have to get your phone out, figure out the best route, then hope you remember all the turns. Or you can use the companion app to reprogram the route, but I've been wary of making changes to the routing on the fly as I've managed to wedge my ELEMNT a couple of times when messing with the routing. So after getting burned pretty badly once and nearly getting stuck without navigation in an unfamiliar area, I only use it in emergencies (e.g. you need to abort and find a short route back home).