I don't think there is any issue with being airborne.
Hilariously, if I told you I had a filed an IFR flight plan, FL150, for you, from Goodwood to Prague, but told you nothing of the route, you could get airborne, fly east, call up London Info/ London Control, carry no paperwork at all, and you would be fine - because ATC tell you where they want you, all the way along. One waypoint at a time, usually, sometimes 2 or 3.
And an astonishing number of old-timers do fly this way. They hack together some route, which usually gets rejected by IFPS, then they refile with REROUTE ACCEPTED and once this is in the system, they depart, knowing nothing of the route that is actually in the system.
The issue is getting the route which you have designed validated in the first place. The SRDs help with that but only if you have lots of time to burn, and I think will deliver success only perhaps 50% of the time (especially on the above route).
IMHO developing a route and getting it validated is desirable because then you know what you should be flying in e.g. the lost comms scenario, and you are in a better position to ask ATC for shortcuts, using waypoints on your route.