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][1]** 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][2]. Still, maintain that vigilance in scanning the sky and making the best of the tools you have, as you've done.


  [1]: https://www.law.cornell.edu/cfr/text/14/91.181
  [2]: https://aviation.stackexchange.com/q/43450/14897