The 500 and 1,000 feet separations you mention, for IFR–VFR and VFR–VFR, respectively, are adequate.
For USA, 14 CFR § 91.181 - Course to be flown is clear, and is echoed in the AIM.
Unless otherwise authorized by ATC, no person may operate an aircraft within controlled airspace under IFR except as follows:
(a) On an ATS route, along the centerline of that airway.
(Emphasis mine.)
Gone are the days of needing passing lights. Still, maintain that vigilance in scanning the sky and making the best of the tools you have, as you've done.
