I'd favour mandatory Mode C/S above some level e.g. 2000ft AGL, for all powered aircraft with an electrical system.
The problem there is that there are a lot of aircraft operating above that height without electrical systems, so the problem will remain.
A lookout doesn't work
But then neither do the technical options either.
TCAS relies on both parties carrying a serviceable transponder and a radar service requires both the infrastructure and sufficient staff - where I am you can whistle for a LARS service at weekends and during the summer holidays